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June 2025 Chemistry Regents — Answer Key & Worked Solutions

Every question from the June 2025 Chemistry Regents exam, with the correct answer and a full step-by-step solution. Check your work, learn the method, then convert your raw score.

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These are real questions from the June 2025 Chemistry New York State Regents examination, published by NYSED. Worked solutions are written by RegentsLab.

Question 1 · 2 pts

Which sequence of historic developments led to the modern model of the atom?

Worked solution
$\text{Dalton: hard sphere}$
$\text{Thomson/Rutherford: mostly empty space (nucleus discovered)}$
$\text{Bohr: electrons in shells outside a nucleus}$
Answer: hard sphere, mostly empty space, electrons in shells outside a nucleus

The historical order of atomic models is Dalton's solid sphere, then Rutherford's mostly empty space with a dense nucleus, then Bohr's electrons in shells.

Question 2 · 2 pts

Which statement compares the energy of an electron in the third shell of a rubidium atom to the energy of an electron in a different shell of the same atom?

Worked solution
$n=3 > n=1 \Rightarrow E_3 > E_1$
Answer: An electron in the third shell has more energy than an electron in the first shell.

Electron energy increases with increasing shell number (principal quantum number), so the third shell has more energy than the first shell.

Question 3 · 2 pts

When an electron in an atom returns from a higher energy state to a lower energy state

Worked solution
$E_{high} \to E_{low} + \Delta E \text{ (emitted photon)}$
Answer: a specific amount of energy is emitted

When an electron drops to a lower energy level, it releases a quantized amount of energy, often as a photon.

Question 4 · 2 pts

The atomic mass of chlorine is the weighted average of the atomic masses of the

Worked solution
$\text{Atomic mass} = \sum (\text{isotope mass} \times \text{natural abundance})$
Answer: naturally occurring isotopes of chlorine

Atomic mass values on the periodic table reflect the weighted average of naturally occurring isotopes of an element.

Question 5 · 2 pts

The elements on the Periodic Table are arranged in order of increasing

Worked solution
$Z: 1, 2, 3, \ldots \text{ increasing left to right}$
Answer: atomic number

The modern periodic table is organized by increasing atomic number (number of protons).

Question 6 · 2 pts

Which element symbol represents a metalloid?

Worked solution
$\text{Te (tellurium) lies along the metalloid staircase on the Periodic Table}$
Answer: Te

Tellurium is located along the zigzag line on the Periodic Table that separates metals from nonmetals, classifying it as a metalloid.

Question 7 · 2 pts

Which term identifies a chemical property that can be used to differentiate a sample of sodium from a sample of gold?

Worked solution
$\text{Chemical property: describes how a substance reacts, e.g., reactivity}$
Answer: reactivity

Reactivity is a chemical property (describes how a substance reacts with other substances), whereas density, mass, and temperature are physical properties.

Question 8 · 2 pts

Which term identifies the type of matter that is composed of two or more different elements chemically combined in a fixed ratio?

Worked solution
$\text{Compound: fixed ratio of elements, chemically combined}$
Answer: compound

A compound is a substance made of two or more elements chemically combined in a fixed, definite ratio.

Question 9 · 2 pts

The coefficients in a balanced equation represent the

Worked solution
$aA + bB \to cC \Rightarrow \text{mole ratio } a:b:c$
Answer: mole ratio of substances in the reaction

Coefficients in a balanced chemical equation indicate the relative number of moles of each reactant and product.

Question 10 · 2 pts

Given the equation representing a reaction: $Br + Br \rightarrow Br_2$. Which statement describes what occurs during this reaction?

Worked solution
$\text{Bond formation: exothermic (releases energy)}$
$\text{Bond breaking: endothermic (absorbs energy)}$
Answer: Energy is released as bonds are formed.

Forming a chemical bond (as two Br atoms combine into Br2) is an exothermic process that releases energy.

Question 11 · 2 pts

Which term represents a property used to determine the degree of polarity in the bond between two atoms?

Worked solution
$\Delta EN = |EN_A - EN_B| \Rightarrow \text{bond polarity}$
Answer: electronegativity

The difference in electronegativity between two bonded atoms determines the degree of polarity of the bond.

Question 12 · 2 pts

Which sample of matter has proportions of components that can be varied?

Worked solution
$\text{Mixtures (e.g., solutions) have variable composition; pure substances do not}$
Answer: aqueous ammonia

Aqueous ammonia is a solution (mixture), so the ratio of ammonia to water can vary; the other choices are pure substances with fixed composition.

Question 13 · 2 pts

Which term identifies a form of energy?

Worked solution
$\text{Electromagnetic energy: a form of energy (e.g., light)}$
Answer: electromagnetic

Electromagnetic (radiant) energy is a form of energy, while pascal, molarity, and temperature are units or measurements, not energy forms.

Question 14 · 2 pts

According to the kinetic molecular theory, which statement describes the particles of an ideal gas?

Worked solution
$\text{KMT assumption: particles undergo random, constant, straight-line motion}$
Answer: The particles move in random, constant, straight-line motion.

The kinetic molecular theory assumes ideal gas particles are in continuous, random, straight-line motion with negligible attractive forces and elastic collisions.

Question 15 · 2 pts

A reaction most likely occurs when particles collide with proper energy and

Worked solution
$\text{Effective collision requires sufficient energy AND proper orientation}$
Answer: orientation

For a reaction to occur, colliding particles need both sufficient energy and the correct orientation (collision theory).

Question 16 · 2 pts

At STP, 2.0 liters of $N_2(g)$ and 2.0 liters of $O_2(g)$ have the same

Worked solution
$\text{Avogadro's principle: equal volumes of gases at same T, P contain equal numbers of molecules}$
Answer: number of molecules

By Avogadro's principle, equal volumes of different gases at the same temperature and pressure (STP) contain the same number of molecules.

Question 17 · 2 pts

Which phrase describes a factor that determines the physical state of a molecular substance?

Worked solution
$\text{Physical state (solid/liquid/gas) depends on particle arrangement and motion}$
Answer: arrangement of the molecules

The physical state of a substance is determined by the arrangement and spacing of its particles (molecules), which affects whether it is solid, liquid, or gas.

Question 18 · 2 pts

Which combination of reactants will result in the fastest rate of reaction of a 1.0-gram sample of Zn(s) and 30. milliliters of HCl(aq) at 25°C?

Worked solution
$\text{Rate} \uparrow \text{ with increased surface area (powder) and increased concentration (2.0 M)}$
Answer: 1.0-g powdered Zn(s) with 2.0 M HCl(aq)

Powdered zinc has more surface area than a cube, and higher acid concentration increases collision frequency, so both factors combined give the fastest rate.

Question 19 · 2 pts

Which term represents the energy absorbed or released during a chemical change?

Worked solution
$\Delta H = H_{products} - H_{reactants}$
Answer: heat of reaction

The heat of reaction is the amount of energy absorbed or released during a chemical reaction.

Question 20 · 2 pts

In terms of energy and disorder, systems in nature have a tendency to undergo changes toward

Worked solution
$\text{Systems favor minimum energy (enthalpy) and maximum entropy (disorder)}$
Answer: lower energy and greater disorder

Natural systems tend to move toward states of lower energy and increased entropy (disorder).

Question 21 · 2 pts

A molecule of which straight chain hydrocarbon contains nine carbon atoms?

Worked solution
$\text{non- prefix} = 9 \text{ carbons}$
Answer: nonane

The prefix "non-" indicates nine carbon atoms, so nonane is the straight chain hydrocarbon with nine carbons.

Question 22 · 2 pts

Which compound is an unsaturated hydrocarbon?

Worked solution
$\text{Alkynes (triple bonds) and alkenes (double bonds) are unsaturated hydrocarbons}$
Answer: 2-butyne

2-butyne is an alkyne, containing a carbon-carbon triple bond, making it an unsaturated hydrocarbon; the others are either alcohols/aldehydes (not hydrocarbons) or saturated (octane).

Question 23 · 2 pts

What is the number of electrons shared between the carbon atoms in an ethene molecule?

Worked solution
$\text{Ethene: } C_2H_4$ Structure is H2C=CH2
$\text{C=C double bond} = 2 \text{ shared pairs}$ Each pair of electrons is 2 electrons
$2 \text{ pairs} \times 2 \text{ electrons/pair} = 4 \text{ electrons shared}$
Answer: 4

Ethene contains a carbon-carbon double bond, which consists of two shared electron pairs, totaling four shared electrons.

Question 24 · 2 pts

Which two terms represent types of organic reactions?

Worked solution
$\text{Solidification and vaporization are phase changes, not organic reactions}$
$\text{Substitution and polymerization are both organic reaction types}$
Answer: substitution and polymerization

Solidification and vaporization are physical phase changes; substitution and polymerization are classic organic reaction types.

Question 25 · 2 pts

In an operating electrolytic cell, oxidation occurs at the

Worked solution
$\text{Oxidation (loss of electrons) always occurs at the anode}$
$\text{This is true in both voltaic and electrolytic cells}$
Answer: anode

By definition, oxidation (electron loss) always occurs at the anode of any electrochemical cell, whether voltaic or electrolytic.

Question 26 · 2 pts

An Arrhenius base yields which negative ions in an aqueous solution?

Worked solution
$\text{Arrhenius base: } \text{yields } OH^-(aq) \text{ as the only negative ion in water}$
Answer: hydroxide ions

By the Arrhenius definition, a base is a substance that produces hydroxide ions, OH-, as the only negative ion when dissolved in water.

Question 27 · 2 pts

Which change in the concentration of $H_3O^+$ ions results in a decrease of one unit in the pH value for an aqueous solution?

Worked solution
$pH = -\log[H_3O^+]$ pH scale is logarithmic base 10
$\text{A decrease of 1 pH unit} \Rightarrow [H_3O^+] \text{ increases by a factor of } 10$
Answer: an increase in the concentration of $H_3O^+$ by a factor of 10

Because pH is a negative log-base-10 scale, each one-unit decrease in pH corresponds to a tenfold increase in hydronium ion concentration.

Question 28 · 2 pts

Based on Table $N$, which phrase describes the half-lives and the decay modes of uranium-235 and uranium-238?

From Reference Table N (Selected Radioisotopes): | Nuclide | Half-Life | Decay Mode | |---|---|---| | U-235 | 7.04 x 10^8 y | alpha | | U-238 | 4.47 x 10^9 y | alpha |

Worked solution
$\text{U-235: half-life} \approx 7.0\times10^8 \text{ y, decay mode: } \alpha$ From Reference Table N
$\text{U-238: half-life} \approx 4.5\times10^9 \text{ y, decay mode: } \alpha$ From Reference Table N
$\text{Half-lives differ but both decay by alpha emission}$
Answer: different half-lives but the same decay mode

Reference Table N lists U-235 and U-238 as both undergoing alpha decay, but their half-lives are very different (about 7.0x10^8 years vs 4.5x10^9 years).

Question 29 · 2 pts

Nuclear fission reactions result in the net conversion of

Worked solution
$E = mc^2$ Einstein mass-energy equivalence
$\text{In fission, a small amount of mass is converted into a large amount of energy}$
Answer: mass to energy

Nuclear fission converts a small amount of mass defect into a large amount of energy, consistent with Einstein's mass-energy equivalence.

Question 30 · 2 pts

What is a risk associated with the production of energy in a nuclear power plant?

Worked solution
$\text{Chemical tracing, radioactive dating, and disease detection are beneficial applications of radioactivity, not risks}$
$\text{Nuclear waste remains radioactive for very long periods, requiring long-term safe storage}$
Answer: long-term storage of waste

Nuclear power plants produce radioactive waste that remains hazardous for extremely long periods, making its long-term storage a major risk of nuclear energy production.

Question 31 · 2 pts

The diagram below represents the path of a subatomic particle as it travels through an electric field between two charged plates. A subatomic particle enters from the left between two horizontal plates and follows a curved path that bends upward toward the positively charged plate (top plate, marked with plus signs) and away from the negatively charged plate (bottom plate, marked with minus signs). Which subatomic particle is traveling between the charged plates?

chem question 31 figure
Worked solution
$\text{The particle path curves toward the positively charged plate}$
$\text{Opposite charges attract, so the particle must be negatively charged}$
$\text{The electron is the negatively charged subatomic particle among the choices}$
Answer: electron

Since the particle's path bends toward the positive plate, it must carry a negative charge; among the choices, only the electron is negatively charged (neutrons are neutral, positrons and protons are positive).

Question 32 · 2 pts

Which electron configuration represents the electrons in a neon atom in an excited state?

Worked solution
$\text{Neon (Z=10) ground state configuration: } 2-8$
$\text{Any configuration for 10 electrons other than 2-8 represents an excited state}$
$2-7-1: 2+7+1=10 \text{ electrons, not the ground-state arrangement}$
Answer: 2-7-1

Neon has 10 electrons with a ground-state configuration of 2-8; the configuration 2-7-1 also totals 10 electrons but is not the lowest-energy (ground state) arrangement, making it an excited state.

Question 33 · 2 pts

Element $X$ reacts with chlorine to form the compound $XCl_3$. In which group on the Periodic Table of the Elements is element $X$ located?

Worked solution
$XCl_3 \Rightarrow X \text{ has an oxidation state of } +3$ Cl has oxidation state -1, three Cl atoms balance a +3 charge
$\text{Group 13 elements typically have a +3 oxidation state}$
Answer: Group 13

Since chlorine has a -1 charge and three chlorine atoms combine with one X atom, X must have a +3 oxidation state, characteristic of Group 13 elements.

Question 34 · 2 pts

What is the number of moles of calcium oxide in a 238-gram sample of CaO? (gram-formula mass of CaO = 56.1 g/mol)

Worked solution
$\text{moles} = \frac{\text{mass}}{\text{gram-formula mass}}$
$\text{moles} = \frac{238\ g}{56.1\ g/mol} \approx 4.24\ mol$
Answer: 4.24 mol

Dividing the given mass by the gram-formula mass of CaO gives the number of moles present in the sample.

Question 35 · 2 pts

What is the molarity of an aqueous solution that contains 0.60 mole of KCl dissolved in 3.5 liters of solution?

Worked solution
$M = \frac{\text{moles solute}}{\text{liters solution}}$
$M = \frac{0.60\ mol}{3.5\ L} \approx 0.17\ M$
Answer: 0.17 M

Molarity is calculated by dividing moles of solute by liters of solution, giving approximately 0.17 M.

Question 36 · 2 pts

Which statement describes the freezing point of a solution prepared by dissolving $NH_4Cl(s)$ in water at 25°C?

Worked solution
$\text{Dissolving a solute in water causes freezing-point depression}$
$\text{Pure water freezes at 0°C; the solution's freezing point is lower than pure water's}$
Answer: The solution has a freezing point lower than 0°C.

Dissolving an ionic solute like NH4Cl in water lowers the freezing point of the solution below that of pure water (0°C), a colligative property called freezing-point depression.

Question 37 · 2 pts

At which temperature and pressure will a sample of a real gas behave most like an ideal gas?

Worked solution
$\text{Real gases behave most ideally at high temperature and low pressure}$
$\text{Among the choices, 1000. K and 0.1 atm has the highest temperature and lowest pressure}$
Answer: 1000. K and 0.1 atm

Real gases most closely approximate ideal gas behavior under conditions of high temperature and low pressure, which minimizes intermolecular attractions and particle volume effects.

Question 38 · 2 pts

In a sealed, rigid cylinder with a movable piston, a sample of $H_2$ gas occupies 30.0 L at 50.0 kPa and 200. K. What is the pressure of the gas if the volume is changed to 60.0 L and the gas is heated to 300. K?

Worked solution
$\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}$ Combined gas law
$P_2 = \frac{P_1V_1T_2}{T_1V_2} = \frac{(50.0)(30.0)(300.)}{(200.)(60.0)}$
$P_2 = \frac{450000}{12000} = 37.5\ kPa$
Answer: 37.5 kPa

Applying the combined gas law with the given initial and final volumes and temperatures gives a final pressure of 37.5 kPa.

Question 39 · 2 pts

Which statement explains why a catalyzed reaction occurs at a faster rate than the same reaction when uncatalyzed?

Worked solution
$\text{A catalyst provides an alternate reaction pathway}$
$\text{This alternate pathway has a lower activation energy, increasing reaction rate}$
Answer: The catalyst provides a different reaction pathway for the reaction with a lower activation energy.

A catalyst speeds up a reaction by offering a new, alternate reaction pathway that has a lower activation energy than the uncatalyzed pathway.

Question 40 · 2 pts

At standard pressure, which 10.0-gram sample of nitrogen has the greatest entropy?

Worked solution
$\text{Entropy (disorder) increases as temperature increases}$
$\text{Among the given temperatures, 90. K is the highest}$
Answer: nitrogen at 90. K

Entropy increases with increasing temperature because particles have more kinetic energy and greater disorder; of the given samples, the one at 90. K has the greatest entropy.

Question 41 · 2 pts

Given the structural formula for an organic compound: a central carbon chain where a nitrogen atom bonded to two hydrogen atoms (H-N-H) is bonded to a carbon atom (bearing one hydrogen and a CH3 branch shown as H-C-H below it), which is in turn bonded to a carbonyl carbon (C=O) that is bonded to an -OH group. Which two functional groups are present in molecules of this compound?

chem question 41 figure
Worked solution
$\text{The } -NH_2 \text{ group attached to a carbon chain (not a carbonyl carbon) is an amine}$
$\text{The } -C(=O)OH \text{ group is a carboxylic (organic) acid group}$
Answer: amine and organic acid

The nitrogen in this structure is bonded to a carbon that is not a carbonyl carbon, making it an amine, while the terminal carbon bonded to both a double-bonded oxygen and a hydroxyl group is a carboxylic (organic) acid group.

Question 42 · 2 pts

Given the equation representing a reaction: $Fe + CuCl_2 \rightarrow FeCl_2 + Cu$. When the iron atoms lose 2 moles of electrons, the copper ions in $CuCl_2$

Worked solution
$Fe \rightarrow Fe^{2+} + 2e^-$ Iron is oxidized, losing 2 moles of electrons
$Cu^{2+} + 2e^- \rightarrow Cu$ Copper ions are reduced, gaining electrons
$\text{Electrons lost must equal electrons gained: 2 moles}$
Answer: gain 2 moles of electrons

In a redox reaction, the number of electrons lost by the oxidized species must equal the number gained by the reduced species; since Fe loses 2 moles of electrons, Cu2+ must gain 2 moles of electrons.

Question 43 · 2 pts

Given the equation representing a reaction: $4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$. What is the change in oxidation state for aluminum in this reaction?

Worked solution
$\text{Al(s) is elemental, oxidation state } = 0$
$\text{In } Al_2O_3, \text{ oxygen is } -2, \text{ so Al must be } +3 \text{ to balance charge}$
Answer: 0 to +3

Elemental aluminum has an oxidation state of 0; in aluminum oxide, Al2O3, aluminum has an oxidation state of +3 to balance the -2 charge of oxygen, so aluminum is oxidized from 0 to +3.

Question 44 · 2 pts

Sulfuric acid, $H_2SO_4(aq)$, can neutralize an aqueous solution of calcium hydroxide, $Ca(OH)_2(aq)$. What is the formula for the salt produced by this neutralization reaction?

Worked solution
$H_2SO_4 + Ca(OH)_2 \rightarrow CaSO_4 + 2H_2O$
$Ca^{2+} \text{ and } SO_4^{2-} \text{ combine in a 1:1 ratio (charges balance)}$
Answer: $CaSO_4$

Neutralization of sulfuric acid with calcium hydroxide produces the salt calcium sulfate, CaSO4, and water, since Ca2+ and SO4 2- combine in a 1:1 ratio to balance charges.

Question 45 · 2 pts

Given the equation representing a reaction: $^{15}_{8}O \rightarrow\ ^{0}_{1}e + \ ^{15}_{7}N$ Which type of reaction is represented by this equation?

Worked solution
$^{15}_{8}O \rightarrow\ ^{0}_{1}e + \ ^{15}_{7}N$ An atom of oxygen-15 emits a positron and becomes an atom of nitrogen-15
$Z: 8 \rightarrow 7$ The atomic number changes, so a new element is formed
Answer: transmutation

Any nuclear change in which one element is converted into a different element (here oxygen becomes nitrogen through positron emission) is classified as transmutation.

Question 46 · 2 pts

Which equation represents nuclear fission?

Worked solution
$$ Choices (1) and (2) are ordinary chemical equations, not nuclear equations
$^{235}_{92}U \rightarrow\ ^{4}_{2}He + \ ^{231}_{90}Th$ Choice (3) is alpha decay of a single nucleus, not fission
$^{235}_{92}U + \ ^{1}_{0}n \rightarrow\ ^{141}_{56}Ba + \ ^{92}_{36}Kr + 3\ ^{1}_{0}n$ A large nucleus struck by a neutron splits into two smaller nuclei plus neutrons: this is fission
Answer: $^{235}_{92}U + \ ^{1}_{0}n \rightarrow\ ^{141}_{56}Ba + \ ^{92}_{36}Kr + 3\ ^{1}_{0}n$

Nuclear fission is the splitting of a large, heavy nucleus into two smaller nuclei (plus neutrons), as shown when a uranium-235 nucleus absorbs a neutron and splits into barium and krypton.

Question 47 · 2 pts

Given the particle-diagram equation shown below, which type of reaction is represented by the equation?

chem question 47 figure
Worked solution
$X_2 + Y_2 \rightarrow 2XY$ Two diatomic elements combine to form molecules containing both elements
$$ Two elements combining to form one compound is the definition of a synthesis (combination) reaction
Answer: synthesis

The diagram shows two different elements, each starting as separate diatomic molecules, combining to form a new compound made of both kinds of atoms, which is a synthesis reaction.

Question 48 · 2 pts

The diagram below represents a laboratory process that is used to separate a mixture of two different liquids. It shows a flask on a hot plate with a thermometer, connected through a rubber stopper to a condenser with water flowing in and out, leading to a beaker collecting liquid. Which process is shown in this diagram?

chem question 48 figure
Worked solution
$$ A flask is heated so one liquid vaporizes; the vapor passes through a water-cooled condenser and re-condenses into a separate collection vessel
$$ Separating liquids based on differences in boiling point using this heat-and-condense setup is distillation
Answer: distillation

Heating a mixture of liquids and condensing the vapor in a separate container to collect a purified liquid is the process of distillation, which separates substances by differences in boiling point.

Question 49 · 2 pts

Given the equation representing a system at equilibrium: $CO_3^{2-}(aq) + H_2O(\ell) \rightleftharpoons HCO_3^-(aq) + OH^-(aq)$ Which statement describes the base in the forward reaction?

Worked solution
$H_2O(\ell) \rightarrow OH^-(aq) + H^+$ Water loses (donates) a hydrogen ion, acting as the acid
$CO_3^{2-}(aq) + H^+ \rightarrow HCO_3^-(aq)$ Carbonate ion gains (accepts) that hydrogen ion, acting as the Bronsted-Lowry base
Answer: The $CO_3^{2-}(aq)$ is the base because it accepts an $H^+$ ion.

In this forward reaction water donates a hydrogen ion (acting as the acid) while carbonate ion accepts that hydrogen ion to become bicarbonate, so carbonate ion is the base.

Question 50 · 2 pts

The graph below shows the decay curve for a 20.0-milligram sample of a radioactive isotope, titled "Decay Curve for a Radioactive Isotope," with Mass (mg) on the vertical axis (0 to 20.0) and Time (d) on the horizontal axis (0 to 40.0+). The curve starts near 20.0 mg at time near 0 and decreases, passing through approximately 10 mg around 8 d, and continuing to decrease toward smaller mass values as time increases. What is the half-life of this isotope?

chem question 50 figure
Worked solution
$20.0\text{ mg} \rightarrow 10.0\text{ mg}$ Reading the graph, the sample decays from 20.0 mg to half that amount, 10.0 mg, at about 8.0 days
$t_{1/2} \approx 8.0\text{ d}$ That elapsed time is the half-life
Answer: 8.0 d

The half-life is the time needed for the mass to drop to half its original value; the curve shows the sample falling from 20.0 mg to 10.0 mg in about 8.0 days.

Question 51 · 1 pt · constructed response

Based on Table F, identify one positive ion that forms an insoluble compound with iodide ions in an aqueous solution.

Model answer: Ag+ (silver ion)
Worked solution
$$ Table F states iodides (I-) are soluble except when combined with certain ions such as Ag+, Pb2+, Cu+, or Hg2^2+
$Ag^+ + I^- \rightarrow AgI(s)$ Silver ion combines with iodide to form insoluble silver iodide
Answer: Ag+ (silver ion)

According to the solubility guidelines in Table F, silver ion (Ag+, also Pb2+, Cu+, or Hg2^2+) forms an insoluble compound with iodide ions.

Question 52 · 1 pt · constructed response

Base your answer on the table below showing the number of subatomic particles in four different atoms, represented by the letters L, M, Q, and R. State the number of valence electrons in an atom of M in the ground state.

chem question 52 figure
Model answer: 5
Worked solution
$Z = 33 \Rightarrow \text{arsenic (As)}$ Atom M has 33 protons, identifying it as arsenic
$\text{Group 15 (VA)} \Rightarrow 5 \text{ valence electrons}$ Arsenic is in Group 15, so it has 5 valence electrons
Answer: 5

Atom M has 33 protons, making it arsenic, an element in Group 15 of the Periodic Table, which has 5 valence electrons.

Question 53 · 1 pt · constructed response

Base your answer on the table below showing the number of subatomic particles in four different atoms, represented by the letters L, M, Q, and R. Compare the mass of a proton in atom R to the mass of an electron in atom R.

chem question 53 figure
Model answer: The mass of a proton in atom R is much greater (about 1836 to 1837 times greater) than the mass of an electron in atom R.
Worked solution
$m_p \approx 1\text{ amu}, \quad m_e \approx 0.0005\text{ amu}$ A proton has approximately 1 atomic mass unit while an electron has negligible mass in comparison
Answer: The mass of a proton in atom R is much greater (about 1836 to 1837 times greater) than the mass of an electron in atom R.

A proton has a mass roughly 1836 times greater than the mass of an electron; the electron's mass is essentially negligible by comparison.

Question 54 · 1 pt · constructed response

Base your answer on the table below showing the number of subatomic particles in four different atoms, represented by the letters L, M, Q, and R. Write the two letters, from this table, that represent atoms that are isotopes of the same element.

chem question 54 figure
Model answer: M and Q
Worked solution
$M: 33p,\ Q: 33p$ Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons
$M: 32n \ne Q: 34n$ M and Q both have 33 protons but differ in neutron number, so they are isotopes of the same element (arsenic)
Answer: M and Q

Atoms M and Q both have 33 protons (same element, arsenic) but different numbers of neutrons (32 vs 34), making them isotopes of each other.

Question 55 · 1 pt · constructed response

Potassium and bromine react to form the ionic compound potassium bromide. Compare the radius of a potassium ion in the ground state to the radius of a potassium atom in the ground state.

Model answer: The radius of a potassium ion (K+) is smaller than the radius of a potassium atom.
Worked solution
$K \rightarrow K^+ + e^-$ A potassium atom loses its single valence electron (from the 4th shell) to form K+
$$ Losing the outermost occupied shell of electrons, combined with the same nuclear charge now pulling fewer electrons, causes the ion's radius to shrink relative to the neutral atom
Answer: The radius of a potassium ion (K+) is smaller than the radius of a potassium atom.

When a potassium atom loses its one valence electron to form K+, it loses its outer occupied electron shell, so the resulting ion has a smaller radius than the neutral atom.

Question 56 · 1 pt · constructed response

Potassium and bromine react to form the ionic compound potassium bromide. Identify the noble gas that has atoms in the ground state with the same electron configuration as a potassium ion, K+, in the ground state.

Model answer: Argon (Ar)
Worked solution
$K^+ : 18\ \text{electrons}$ A potassium atom (19 electrons) loses one electron to form K+, which has 18 electrons
$Ar: 18\ \text{electrons}$ Argon, atomic number 18, has the same 2-8-8 electron configuration as K+
Answer: Argon (Ar)

A potassium ion (K+) has 18 electrons with configuration 2-8-8, which is identical to the ground-state electron configuration of the noble gas argon.

Question 57 · 1 pt · constructed response

In the space in your answer booklet, draw a Lewis electron-dot diagram of a bromide ion, $Br^-$.

Model answer: A bromide ion is shown as the symbol Br surrounded by four pairs of dots (8 valence electrons total, representing a complete octet), enclosed in brackets with a superscript negative charge: $[:\ddot{\ddot{B}r}\ddot{\ddot{:}}]^{-}$
Worked solution
$Br: 7\ \text{valence electrons}$ A neutral bromine atom has 7 valence electrons
$Br + e^- \rightarrow Br^-$ Bromine gains one electron to form the bromide ion
$Br^-: 8\ \text{valence electrons}$ The bromide ion has 8 valence electrons, a complete octet, shown as 4 pairs of dots around the Br symbol inside brackets with a negative charge
Answer: A bromide ion is shown as the symbol Br surrounded by four pairs of dots (8 valence electrons total, representing a complete octet), enclosed in brackets with a superscript negative charge: $[:\ddot{\ddot{B}r}\ddot{\ddot{:}}]^{-}$

Bromine has 7 valence electrons and gains one more to form Br-, so the correct Lewis dot diagram shows the Br symbol surrounded by 4 lone pairs (8 dots) enclosed in brackets with a 1- charge.

Question 58 · 1 pt · constructed response

Potassium and bromine react to form the ionic compound potassium bromide. Identify the type of bonding in a sample of solid potassium.

Model answer: Metallic bonding
Worked solution
$$ Solid potassium is a single metallic element, so its atoms are held together by a sea of mobile (delocalized) valence electrons surrounding positive metal ions
Answer: Metallic bonding

A sample of pure solid potassium consists only of metal atoms held together by metallic bonding, in which valence electrons are delocalized among the potassium ions.

Question 59 · 1 pt · constructed response

Hydrogen sulfide, $H_2S$, and water, $H_2O$, are both molecular compounds. State, in terms of valence electrons, why the bonds in a molecule of hydrogen sulfide are covalent.

Model answer: The H and S atoms share pairs of valence electrons to form the bonds, rather than transferring electrons from one atom to another.
Worked solution
$$ Both hydrogen and sulfur are nonmetals; nonmetal atoms bonded together share valence electrons to each attain a stable (filled) outer shell
Answer: The H and S atoms share pairs of valence electrons to form the bonds, rather than transferring electrons from one atom to another.

In H2S, the hydrogen and sulfur atoms are nonmetals that achieve stable electron configurations by sharing pairs of valence electrons, which is what makes the bonds covalent rather than ionic.

Question 60 · 1 pt · constructed response

Hydrogen sulfide, $H_2S$, and water, $H_2O$, are both molecular compounds. Explain, in terms of charge distribution, why a molecule of $H_2S$ is a polar molecule.

Model answer: The H2S molecule has an unequal (asymmetrical) distribution of charge because its bent shape and the electronegativity difference between S and H leave the sulfur end of the molecule more negative and the hydrogen ends more positive, so the bond polarities do not cancel.
Worked solution
$$ H2S has a bent molecular shape, so the two polar H-S bonds do not point in opposite directions
$$ Because the bond dipoles do not cancel, there is an unequal (uneven) distribution of charge across the molecule, giving it a net dipole
Answer: The H2S molecule has an unequal (asymmetrical) distribution of charge because its bent shape and the electronegativity difference between S and H leave the sulfur end of the molecule more negative and the hydrogen ends more positive, so the bond polarities do not cancel.

Because H2S is bent rather than linear, the polar H-S bonds do not cancel each other, leaving an uneven distribution of charge across the molecule and making it polar.

Question 61 · 1 pt · constructed response

An equilibrium system in a sealed, rigid container is represented by the equation below. $H_2(g) + I_2(g) + heat \rightleftharpoons 2HI(g)$ Compare the rate of the forward reaction to the rate of the reverse reaction in this system.

Model answer: The rate of the forward reaction is equal to the rate of the reverse reaction.
Worked solution
$rate_{forward} = rate_{reverse}$ By definition, a system at equilibrium has the forward and reverse reaction rates equal
Answer: The rate of the forward reaction is equal to the rate of the reverse reaction.

At chemical equilibrium, the forward and reverse reactions occur at equal rates, which is why the concentrations of reactants and products remain constant.

Question 62 · 1 pt · constructed response

An equilibrium system in a sealed, rigid container is represented by the equation below. $H_2(g) + I_2(g) + heat \rightleftharpoons 2HI(g)$ State the effect on the concentration of iodine gas when hydrogen gas is added to this equilibrium system.

Model answer: The concentration of iodine gas decreases.
Worked solution
$$ Adding H2 increases the concentration of a reactant, shifting the equilibrium toward the products (forward direction) by Le Chatelier's principle
$H_2 + I_2 \rightarrow 2HI$ As the forward reaction proceeds to re-establish equilibrium, I2 is consumed, so its concentration decreases
Answer: The concentration of iodine gas decreases.

Adding H2 shifts the equilibrium to the right according to Le Chatelier's principle, consuming I2 in the process and decreasing its concentration.

Question 63 · 1 pt · constructed response

An equilibrium system in a sealed, rigid container is represented by the equation below. $H_2(g) + I_2(g) + heat \rightleftharpoons 2HI(g)$ Draw a potential energy diagram on the labeled axes in your answer booklet for the forward reaction.

chem question 63 figure
Model answer: A potential energy diagram in which the products (2HI) are drawn at a higher potential energy than the reactants (H2 + I2), with a peak (activated complex) above the products between them, since heat is written as a reactant, meaning the forward reaction is endothermic.
Worked solution
$H_2(g) + I_2(g) + heat \rightleftharpoons 2HI(g)$ Heat is on the reactant side, so the forward reaction absorbs heat: it is endothermic
$PE_{products} > PE_{reactants}$ On the diagram, the reactants' PE line starts lower, rises to a peak (representing the activated complex, at the height of the activation energy), then drops down to the products' PE line, which is higher than the reactants' PE line
Answer: A potential energy diagram in which the products (2HI) are drawn at a higher potential energy than the reactants (H2 + I2), with a peak (activated complex) above the products between them, since heat is written as a reactant, meaning the forward reaction is endothermic.

Since heat appears as a reactant, the forward reaction is endothermic, so the correct potential energy diagram shows the reactants starting low, rising over an activation-energy peak, and ending at a products energy level that is higher than the reactants' energy level.

Question 64 · 1 pt · constructed response

Radon-222 and radon-223 are two radioactive isotopes of radon. Radon-222 emits alpha particles when it decays and radon-223 emits beta particles when it decays. Complete the nuclear equation in your answer booklet for the decay of radon-222 by writing a notation for the missing product.

Model answer: $^{218}_{84}Po$ (polonium-218)
Worked solution
$^{222}_{86}Rn \rightarrow\ ^{4}_{2}He + \ ^{218}_{84}Po$ Alpha decay releases a helium-4 nucleus (alpha particle); mass number decreases by 4 (222-4=218) and atomic number decreases by 2 (86-2=84)
Answer: $^{218}_{84}Po$ (polonium-218)

In alpha decay, radon-222 (Z=86, A=222) emits an alpha particle (helium-4 nucleus), leaving a daughter nucleus with mass number 218 and atomic number 84, which is polonium-218.

Question 65 · 1 pt · constructed response

Radon-222 and radon-223 are two radioactive isotopes of radon. Radon-222 emits alpha particles when it decays and radon-223 emits beta particles when it decays. Determine the time required for a sample of radon-222 to decay until only $\frac{1}{4}$ of the original sample remains unchanged.

Model answer: 7.64 days
Worked solution
$t_{1/2}(Rn\text{-}222) = 3.82\text{ d}$ From Table N, the half-life of radon-222 is 3.82 days
$1 \rightarrow \frac{1}{2} \rightarrow \frac{1}{4}$ Decaying to 1/4 of the original amount takes exactly 2 half-lives
$2 \times 3.82\text{ d} = 7.64\text{ d}$ Multiply the half-life by 2 to get the total elapsed time
Answer: 7.64 days

Since 1/4 remaining corresponds to exactly two half-lives, and the half-life of radon-222 (from Table N) is 3.82 days, the total time elapsed is 2 x 3.82 = 7.64 days.

Question 66 · 1 pt · constructed response

Chlorine, phosphorus, and sulfur are three elements located in Period 3 on the Periodic Table that have been used to produce important compounds for agriculture. Identify the element classification of these three elements.

Model answer: Nonmetals
Worked solution
$$ Chlorine, phosphorus, and sulfur are all located to the right side of the Period 3 staircase on the Periodic Table, in the nonmetal region
Answer: Nonmetals

Chlorine, phosphorus, and sulfur are all classified as nonmetals on the Periodic Table.

Question 67 · 1 pt · constructed response

Base your answers to questions 67 through 69 on the information below and on your knowledge of chemistry. Chlorine, phosphorus, and sulfur are three elements located in Period 3 on the Periodic Table that have been used to produce important compounds for agriculture. 67 Determine the mass number of an atom of sulfur that contains 18 neutrons.

Model answer: 34
Worked solution
$\text{Atomic number of S} = 16 \text{ (number of protons)}$
$\text{Mass number} = \text{protons} + \text{neutrons} = 16 + 18 = 34$
Answer: 34

The mass number of an atom equals the sum of its protons and neutrons. Sulfur's atomic number (16 protons) plus the given 18 neutrons gives a mass number of 34.

Question 68 · 1 pt · constructed response

Base your answers to questions 67 through 69 on the information below and on your knowledge of chemistry. Chlorine, phosphorus, and sulfur are three elements located in Period 3 on the Periodic Table that have been used to produce important compounds for agriculture. 68 State the trend in atomic radius as these three Period 3 elements on the Periodic Table are considered in order of increasing atomic number.

Model answer: Atomic radius decreases as atomic number increases across Period 3 (from phosphorus to sulfur to chlorine).
Worked solution
$\text{Increasing atomic number: } P(15) \rightarrow S(16) \rightarrow Cl(17)$
$\text{Nuclear charge increases while shielding stays constant, pulling electrons closer}$
Answer: Atomic radius decreases as atomic number increases across Period 3 (from phosphorus to sulfur to chlorine).

Moving left to right across a period, increasing nuclear charge pulls the same-shell electrons in more tightly, so atomic radius decreases.

Question 69 · 1 pt · constructed response

Base your answers to questions 67 through 69 on the information below and on your knowledge of chemistry. Chlorine, phosphorus, and sulfur are three elements located in Period 3 on the Periodic Table that have been used to produce important compounds for agriculture. 69 Identify which of these three elements is a gas at STP.

Model answer: Chlorine
Worked solution
$\text{Reference Table S lists the standard state of each element at STP}$
$\text{Cl}_2 \text{ is listed as a gas; P and S are listed as solids}$
Answer: Chlorine

According to the Periodic Table reference tables, chlorine exists as a diatomic gas at standard temperature and pressure, while phosphorus and sulfur are solids.

Question 70 · 1 pt · constructed response

Base your answers to questions 70 through 72 on the information below and on your knowledge of chemistry. Silicon carbide, SiC, is a compound used for making sandpaper and for cutting and polishing metals and glass. The balanced equation below represents the production of silicon carbide. $SiO_2 + 3C + \text{heat} \rightarrow SiC + 2CO$ 70 Show a numerical setup for calculating the gram-formula mass of silicon dioxide.

Model answer: $28.1 + 2(16.0) = 60.1$ g/mol
Worked solution
$\text{Si} = 28.1\ \text{g/mol}, \quad \text{O} = 16.0\ \text{g/mol}$
$28.1 + 2(16.0) = 60.1\ \text{g/mol}$
Answer: $28.1 + 2(16.0) = 60.1$ g/mol

The gram-formula mass of SiO2 is found by adding the atomic mass of silicon to twice the atomic mass of oxygen.

Question 71 · 1 pt · constructed response

Base your answers to questions 70 through 72 on the information below and on your knowledge of chemistry. Silicon carbide, SiC, is a compound used for making sandpaper and for cutting and polishing metals and glass. The balanced equation below represents the production of silicon carbide. $SiO_2 + 3C + \text{heat} \rightarrow SiC + 2CO$ 71 Write a chemical name for the product, CO, in the equation.

Model answer: Carbon monoxide
Worked solution
$CO \rightarrow \text{one carbon, one oxygen, use mono- prefix for single oxygen}$
Answer: Carbon monoxide

CO is a binary molecular compound named using prefixes; with one carbon and one oxygen atom, it is named carbon monoxide.

Question 72 · 1 pt · constructed response

Base your answers to questions 70 through 72 on the information below and on your knowledge of chemistry. Silicon carbide, SiC, is a compound used for making sandpaper and for cutting and polishing metals and glass. The balanced equation below represents the production of silicon carbide. $SiO_2 + 3C + \text{heat} \rightarrow SiC + 2CO$ 72 Determine the number of moles of carbon that are required to produce 4.0 moles of silicon carbide, SiC.

Model answer: 12 mol C
Worked solution
$\text{Mole ratio: } 3\ \text{mol C} : 1\ \text{mol SiC}$
$4.0\ \text{mol SiC} \times \frac{3\ \text{mol C}}{1\ \text{mol SiC}} = 12\ \text{mol C}$
Answer: 12 mol C

Using the mole ratio from the balanced equation (3 mol C per 1 mol SiC), 4.0 moles of SiC requires 12 moles of carbon.

Question 73 · 1 pt · constructed response

Base your answers to questions 73 through 75 on the information below and on your knowledge of chemistry. At standard pressure, a student heated a 120.-gram sample of H2O(s) at -25°C until the entire sample boiled for four minutes. This process is represented by the heating curve below. During this laboratory activity appropriate safety equipment was used and safety procedures were followed. 73 Identify the physical change that takes place during interval BC of the heating curve.

chem question 73 figure
Model answer: Melting (fusion): solid H2O changes to liquid H2O
Worked solution
$\text{Interval } BC \text{ is flat at } 0^{\circ}C \Rightarrow \text{constant temperature phase change}$
$\text{At } 0^{\circ}C,\ H_2O(s) \rightarrow H_2O(l)$
Answer: Melting (fusion): solid H2O changes to liquid H2O

A flat segment on a heating curve indicates a phase change at constant temperature; at 0 degrees Celsius this is melting of ice to liquid water.

Question 74 · 1 pt · constructed response

Base your answers to questions 73 through 75 on the information below and on your knowledge of chemistry. At standard pressure, a student heated a 120.-gram sample of H2O(s) at -25°C until the entire sample boiled for four minutes. This process is represented by the heating curve below. During this laboratory activity appropriate safety equipment was used and safety procedures were followed. 74 State what happens to the average kinetic energy of the molecules in the H2O sample during interval CD.

chem question 74 figure
Model answer: The average kinetic energy of the molecules increases.
Worked solution
$\text{Interval } CD: \text{temperature rises from } 0^{\circ}C \text{ to } 100^{\circ}C$
$\text{Temperature is a measure of average kinetic energy, so KE increases}$
Answer: The average kinetic energy of the molecules increases.

Since temperature is a direct measure of average kinetic energy, and interval CD shows a rising temperature (liquid water warming), the average kinetic energy of the molecules increases.

Question 75 · 1 pt · constructed response

Base your answers to questions 73 through 75 on the information below and on your knowledge of chemistry. At standard pressure, a student heated a 120.-gram sample of H2O(s) at -25°C until the entire sample boiled for four minutes. This process is represented by the heating curve below. During this laboratory activity appropriate safety equipment was used and safety procedures were followed. 75 Determine the quantity of heat absorbed when the 120.-gram sample of water boils away completely at its boiling point.

Model answer: $2.7 \times 10^{5}$ J (271,200 J)
Worked solution
$q = m \times H_v$
$H_v(\text{water}) = 2260\ \text{J/g (Reference Table)}$
$q = 120.\ \text{g} \times 2260\ \text{J/g} = 271{,}200\ \text{J} \approx 2.7 \times 10^{5}\ \text{J}$
Answer: $2.7 \times 10^{5}$ J (271,200 J)

The heat absorbed during boiling (a phase change at constant temperature) is calculated using q = m×Hv, where Hv is the heat of vaporization of water from the reference tables, 2260 J/g.

Question 76 · 1 pt · constructed response

Base your answers to questions 76 through 78 on the information below and on your knowledge of chemistry. The structural formulas of two of the three isomers of pentane and their boiling points at standard pressure are shown below. 76 Write the general formula of the homologous series to which these compounds belong.

chem question 76 figure
Model answer: $C_nH_{2n+2}$
Worked solution
$\text{Pentane and dimethylpropane are both alkanes (isomers of } C_5H_{12})$
$\text{General formula for alkanes: } C_nH_{2n+2}$
Answer: $C_nH_{2n+2}$

Pentane and dimethylpropane are both saturated hydrocarbons (alkanes) with only single bonds, which belong to the homologous series with general formula CnH2n+2.

Question 77 · 1 pt · constructed response

Base your answers to questions 76 through 78 on the information below and on your knowledge of chemistry. The structural formulas of two of the three isomers of pentane and their boiling points at standard pressure are shown below (pentane, boiling point 36.1°C; dimethylpropane, boiling point 9.5°C). 77 State, in terms of strength of intermolecular forces, why the boiling point of dimethylpropane is lower than the boiling point of pentane.

chem question 77 figure
Model answer: Dimethylpropane has weaker intermolecular forces than pentane because its more compact, branched shape reduces surface contact between molecules.
Worked solution
$\text{Dimethylpropane is more branched/compact than straight-chain pentane}$
$\text{Less surface area contact} \Rightarrow \text{weaker London dispersion forces} \Rightarrow \text{lower boiling point}$
Answer: Dimethylpropane has weaker intermolecular forces than pentane because its more compact, branched shape reduces surface contact between molecules.

Both are nonpolar molecules held together by London dispersion forces. The more spherical, branched shape of dimethylpropane has less surface area in contact between neighboring molecules than the straight chain of pentane, resulting in weaker intermolecular forces and thus a lower boiling point.

Question 78 · 1 pt · constructed response

Base your answers to questions 76 through 78 on the information below and on your knowledge of chemistry. The structural formulas of two of the three isomers of pentane and their boiling points at standard pressure are shown (pentane and dimethylpropane). 78 In the space in your answer booklet, draw a structural formula of the third pentane isomer, 2-methylbutane.

chem question 78 figure
Model answer: A four-carbon main chain (butane) with a methyl (CH3) branch on the second carbon: CH3-CH(CH3)-CH2-CH3
Worked solution
$\text{Main chain: } C-C-C-C \text{ (butane backbone)}$
$\text{Attach a } CH_3 \text{ branch to the second carbon}$
$CH_3-CH(CH_3)-CH_2-CH_3,\ \text{molecular formula } C_5H_{12}$
Answer: A four-carbon main chain (butane) with a methyl (CH3) branch on the second carbon: CH3-CH(CH3)-CH2-CH3

2-methylbutane is drawn as a four-carbon chain (butane) with a methyl group branching off the second carbon, giving each carbon the correct number of hydrogens to satisfy four bonds, and the overall molecular formula C5H12.

Question 79 · 1 pt · constructed response

Base your answers to questions 79 through 81 on the information below and on your knowledge of chemistry. In a laboratory activity, a student constructs an electrochemical cell. The diagram and the ionic equation below represent this cell and the reaction that occurs. During this laboratory activity appropriate safety equipment is used and safety procedures are followed. Voltaic Cell diagram: A voltmeter (V) connects via wires to a Zn(s) electrode in a beaker of Zn2+(aq) and a Mg(s) electrode in a beaker of Mg2+(aq), joined by a salt bridge. $Zn^{2+}(aq) + Mg(s) \rightarrow Zn(s) + Mg^{2+}(aq)$ 79 State the form of energy that is converted to electrical energy in this type of cell.

chem question 79 figure
Model answer: Chemical energy
Worked solution
$\text{Voltaic (galvanic) cells convert chemical energy from a spontaneous redox reaction into electrical energy}$
Answer: Chemical energy

A voltaic cell uses a spontaneous chemical reaction (the redox reaction between Zn2+ and Mg) to generate electricity, converting chemical energy into electrical energy.

Question 80 · 1 pt · constructed response

Base your answers to questions 79 through 81 on the information below and on your knowledge of chemistry. In a laboratory activity, a student constructs an electrochemical cell represented by the ionic equation: $Zn^{2+}(aq) + Mg(s) \rightarrow Zn(s) + Mg^{2+}(aq)$ The cell has a Zn(s) electrode in Zn2+(aq) and a Mg(s) electrode in Mg2+(aq), connected by wires through a voltmeter, with a salt bridge joining the two half-cells. 80 State, in terms of the electrodes, the direction of electron flow in the external circuit when the cell is operating.

Model answer: Electrons flow through the external circuit from the Mg electrode to the Zn electrode.
Worked solution
$Mg(s) \rightarrow Mg^{2+}(aq) + 2e^- \quad \text{(oxidation at Mg electrode, anode)}$
$Zn^{2+}(aq) + 2e^- \rightarrow Zn(s) \quad \text{(reduction at Zn electrode, cathode)}$
$\text{Electrons flow externally from anode (Mg) to cathode (Zn)}$
Answer: Electrons flow through the external circuit from the Mg electrode to the Zn electrode.

Mg is oxidized and loses electrons at the Mg electrode (anode), while Zn2+ is reduced at the Zn electrode (cathode); electrons therefore flow through the external wire from the Mg electrode to the Zn electrode.

Question 81 · 1 pt · constructed response

Base your answers to questions 79 through 81 on the information below and on your knowledge of chemistry. The cell is represented by the ionic equation: $Zn^{2+}(aq) + Mg(s) \rightarrow Zn(s) + Mg^{2+}(aq)$ 81 Write a balanced equation for the reduction half-reaction that occurs in this cell.

Model answer: $Zn^{2+}(aq) + 2e^- \rightarrow Zn(s)$
Worked solution
$\text{Reduction is the gain of electrons}$
$Zn^{2+} \text{ gains 2 electrons to become } Zn(s)$
$Zn^{2+}(aq) + 2e^- \rightarrow Zn(s)$
Answer: $Zn^{2+}(aq) + 2e^- \rightarrow Zn(s)$

Reduction involves a gain of electrons; in this cell Zn2+ ions gain two electrons to form solid zinc, which is the reduction half-reaction.

Question 82 · 1 pt · constructed response

Base your answers to questions 82 through 85 on the information below and on your knowledge of chemistry. During a titration, 11.1 mL of hydrochloric acid, HCl(aq), is exactly neutralized by 33.3 mL of 0.10 M sodium hydroxide, NaOH(aq). During this laboratory activity, appropriate safety equipment is used and safety procedures are followed. 82 Identify the positive ion present in the HCl(aq).

Model answer: Hydrogen ion, $H^+$ (hydronium ion, $H_3O^+$)
Worked solution
$HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$
Answer: Hydrogen ion, $H^+$ (hydronium ion, $H_3O^+$)

HCl is an Arrhenius acid that dissociates in water to yield hydrogen ions (commonly represented as hydronium ions, H3O+) and chloride ions.

Question 83 · 1 pt · constructed response

Base your answers to questions 82 through 85 on the information below and on your knowledge of chemistry. During a titration, 11.1 mL of hydrochloric acid, HCl(aq), is exactly neutralized by 33.3 mL of 0.10 M sodium hydroxide, NaOH(aq). During this laboratory activity, appropriate safety equipment is used and safety procedures are followed. 83 Determine the concentration of the HCl(aq) using the titration data.

Model answer: 0.30 M
Worked solution
$M_{acid}V_{acid} = M_{base}V_{base}$
$M_{HCl}(11.1\ mL) = (0.10\ M)(33.3\ mL)$
$M_{HCl} = \frac{(0.10)(33.3)}{11.1} = 0.30\ M$
Answer: 0.30 M

Using the titration relationship MacidVacid = MbaseVbase and solving for the unknown acid concentration gives 0.30 M HCl.

Question 84 · 1 pt · constructed response

Base your answers to questions 82 through 85 on the information below and on your knowledge of chemistry. During a titration, 11.1 mL of hydrochloric acid, HCl(aq), is exactly neutralized by 33.3 mL of 0.10 M sodium hydroxide, NaOH(aq). During this laboratory activity, appropriate safety equipment is used and safety procedures are followed. 84 State the color of bromcresol green if it is added to a sample of the sodium hydroxide solution.

Model answer: Blue
Worked solution
$\text{NaOH(aq) is basic (pH} > 7)$
$\text{Reference Table M: bromcresol green is blue in its basic range (above pH 5.4)}$
Answer: Blue

Sodium hydroxide solution is basic, and according to the indicator reference table, bromcresol green turns blue in basic solutions.

Question 85 · 1 pt · constructed response

Base your answers to questions 82 through 85 on the information below and on your knowledge of chemistry. During a titration, 11.1 mL of hydrochloric acid, HCl(aq), is exactly neutralized by 33.3 mL of 0.10 M sodium hydroxide, NaOH(aq). During this laboratory activity, appropriate safety equipment is used and safety procedures are followed. 85 State, in terms of ions, why a sample of the 0.10 M NaOH is a good conductor of electricity.

Model answer: NaOH dissociates completely in water into freely moving Na+ and OH- ions, which carry electric charge through the solution.
Worked solution
$NaOH(s) \rightarrow Na^+(aq) + OH^-(aq)$
$\text{Mobile, charged ions in solution allow it to conduct electricity}$
Answer: NaOH dissociates completely in water into freely moving Na+ and OH- ions, which carry electric charge through the solution.

NaOH is a strong electrolyte that dissociates completely into Na+ and OH- ions in water; these freely moving, charged ions allow the solution to conduct an electric current.

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