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OSAT Chemistry practice questions

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Question 1Chemistry content domain (not enumerated in source)

Which statement correctly describes the arrangement of particles in a solid at the particle level?

  1. Particles are closely packed in fixed positions and vibrate about those positions. Correct answer.
  2. Particles are closely packed but move freely past one another throughout the sample.
  3. Particles are ionized into a plasma of free electrons and positive ions.
  4. Particles are far apart and move rapidly in straight lines with little interaction.

Explanation

In a solid, intermolecular or ionic forces hold particles in fixed lattice positions, so they can only vibrate in place rather than translate. This restricted motion explains the definite shape and volume of solids. Free sliding characterizes liquids, and large separations characterize gases.

Question 2Chemistry content domain (not enumerated in source)

An atom of an element has the electron configuration 1s² 2s² 2p⁶ 3s². To which group of the periodic table does this element belong?

  1. Group 2 Correct answer.
  2. Group 18
  3. Group 12
  4. Group 16

Explanation

The configuration ends in 3s², so the element has two valence electrons in the s subshell of period 3, which places it in Group 2 (the alkaline earth metals). Magnesium fits this configuration. Group 12 would require a filled d subshell, and Groups 16 and 18 require p electrons.

Question 3Chemistry content domain (not enumerated in source)

Which type of bond results from the electrostatic attraction between oppositely charged ions?

  1. Hydrogen bonding
  2. Covalent bonding
  3. Metallic bonding
  4. Ionic bonding Correct answer.

Explanation

Ionic bonding arises when electrons are transferred from a metal to a nonmetal, creating cations and anions that attract electrostatically. Covalent bonds share electrons, metallic bonds delocalize electrons among cations, and hydrogen bonds are weaker intermolecular attractions.

Question 4Chemistry content domain (not enumerated in source)

In the balanced equation 2H₂(g) + O₂(g) → 2H₂O(g), how many moles of water form when 3.0 mol of O₂ reacts completely with excess H₂?

  1. 3.0 mol
  2. 6.0 mol Correct answer.
  3. 12.0 mol
  4. 1.5 mol

Explanation

The stoichiometric ratio of H₂O to O₂ is 2:1, so each mole of O₂ produces two moles of water. Multiplying 3.0 mol O₂ by 2 gives 6.0 mol H₂O. Hydrogen is in excess, so oxygen is the limiting reactant that sets the yield.

Question 5Chemistry content domain (not enumerated in source)

Which property is characteristic of an Arrhenius acid in aqueous solution?

  1. It increases the hydroxide ion concentration.
  2. It increases the hydrogen ion concentration. Correct answer.
  3. It donates an electron pair to a proton.
  4. It accepts a proton from water.

Explanation

An Arrhenius acid is a substance that produces hydrogen ions (protons) when dissolved in water, raising [H⁺]. A base instead raises [OH⁻]. Proton acceptance and electron-pair donation describe Brønsted and Lewis bases, respectively.

Question 6Chemistry content domain (not enumerated in source)

Which change increases the rate of a gas-phase reaction according to collision theory?

  1. Increasing the volume of the reaction vessel
  2. Removing a catalyst from the reaction mixture
  3. Adding an inert gas at constant volume
  4. Raising the temperature of the reaction mixture Correct answer.

Explanation

Raising temperature increases the average kinetic energy of molecules, so a larger fraction of collisions exceeds the activation energy, increasing reaction rate. Adding inert gas at constant volume does not change reactant concentrations, and expanding the volume lowers concentrations.

Question 7Chemistry content domain (not enumerated in source)

Which statement best describes the first law of thermodynamics as applied to a chemical system?

  1. The enthalpy change of a reaction equals the heat released at constant pressure.
  2. The entropy of the universe always increases in a spontaneous process.
  3. Energy can be converted between forms but cannot be created or destroyed. Correct answer.
  4. A system at equilibrium has equal forward and reverse rate constants.

Explanation

The first law is conservation of energy: energy may change form or transfer between system and surroundings, but the total is constant. For a closed system, ΔU = q + w. The other statements concern entropy, enthalpy definitions, or equilibrium kinetics rather than energy conservation.

Question 8Chemistry content domain (not enumerated in source)

Which quantum number determines the shape of an atomic orbital?

  1. The principal quantum number, n
  2. The azimuthal quantum number, ℓ Correct answer.
  3. The magnetic quantum number, mℓ
  4. The spin quantum number, ms

Explanation

The azimuthal (angular momentum) quantum number ℓ determines the subshell type and therefore the orbital shape (s, p, d, f). The principal quantum number sets overall size and energy level, mℓ sets orientation, and ms sets spin.

Question 9Chemistry content domain (not enumerated in source)

How many significant figures are in the measured value 0.004560 m?

  1. 3
  2. 7
  3. 5
  4. 4 Correct answer.

Explanation

Leading zeros before the first nonzero digit are not significant, so the zeros in 0.004 are placeholders. The digits 4, 5, 6, and the trailing zero after a decimal are significant, giving four significant figures.

Question 10Chemistry content domain (not enumerated in source)

A sample of gas occupies 2.0 L at 300 K. At constant pressure, what volume does it occupy at 600 K?

  1. 2.0 L
  2. 1.0 L
  3. 4.0 L Correct answer.
  4. 6.0 L

Explanation

Charles's law states that volume is directly proportional to absolute temperature at constant pressure: V₂ = V₁(T₂/T₁). Substituting gives 2.0 L × (600 K/300 K) = 4.0 L. Doubling the Kelvin temperature doubles the volume.

Question 11Chemistry content domain (not enumerated in source)

In acidic solution, reduction of MnO4^- to Mn2+ changes manganese's oxidation state by how many units?

  1. A decrease of seven
  2. A decrease of five Correct answer.
  3. An increase of five
  4. A decrease of two

Explanation

Oxygen contributes minus eight in permanganate, so manganese starts at plus seven. The product is plus two, making the oxidation-state change a decrease of five units per manganese atom.

Question 12Chemistry content domain (not enumerated in source)

Which compound exhibits hydrogen bonding as a significant intermolecular force in the pure liquid?

  1. CO₂
  2. H₂S
  3. NH₃ Correct answer.
  4. CH₄

Explanation

Hydrogen bonding requires hydrogen covalently bonded to a highly electronegative N, O, or F atom. Ammonia has N–H bonds, so its molecules form hydrogen bonds. Methane, hydrogen sulfide, and carbon dioxide lack this combination.

Question 13Chemistry content domain (not enumerated in source)

A reaction has ΔH = +58 kJ and ΔS = +175 J/K. At approximately what temperature does the reaction become spontaneous at standard conditions?

  1. The reaction is never spontaneous
  2. About 3 K
  3. About 0.33 K
  4. About 331 K Correct answer.

Explanation

Spontaneity requires ΔG = ΔH − TΔS < 0. Solving T > ΔH/ΔS with ΔH in joules gives T > 58,000 J / 175 J/K ≈ 331 K, so the reaction becomes spontaneous above about 331 K. Because ΔH is positive and ΔS is positive, spontaneity requires a sufficiently high temperature.

Question 14Chemistry content domain (not enumerated in source)

Which factor increases the solubility of a slightly soluble ionic salt whose dissolution is endothermic?

  1. Adding a common ion to the solution
  2. Decreasing the temperature of the solution
  3. Increasing the temperature of the solution Correct answer.
  4. Increasing the pressure above the solution

Explanation

For an endothermic dissolution, heat acts as a reactant, so raising temperature shifts the equilibrium toward dissolved ions according to Le Châtelier's principle. A common ion would decrease solubility by shifting equilibrium left. Pressure has negligible effect on condensed-phase solubility.

Question 15Chemistry content domain (not enumerated in source)

Which species acts as the Lewis acid in the reaction BF₃ + NH₃ → F₃B–NH₃?

  1. BF₃ Correct answer.
  2. F₃B–NH₃
  3. Neither reactant; this is not a Lewis acid-base reaction
  4. NH₃

Explanation

A Lewis acid is an electron-pair acceptor. Boron trifluoride has an incomplete octet and accepts the lone pair from nitrogen, so BF₃ is the Lewis acid while NH₃ is the Lewis base. The product is a Lewis adduct.

Question 16Chemistry content domain (not enumerated in source)

Which statement about a reaction at chemical equilibrium is correct?

  1. The concentrations of reactants and products are equal.
  2. The forward and reverse reaction rates are equal. Correct answer.
  3. The reaction has stopped in both directions.
  4. The value of the equilibrium constant changes as concentrations change.

Explanation

At equilibrium, the rates of the forward and reverse reactions are equal, so concentrations remain constant over time even though both reactions continue. Concentrations need not be equal, and the equilibrium constant depends only on temperature for a given reaction.

Question 17Chemistry content domain (not enumerated in source)

Which of the following is the correct electron configuration for a neutral chromium atom (Z = 24)?

  1. [Ar] 4s² 3d⁴
  2. [Ar] 4s² 3d⁶
  3. [Ar] 4s¹ 3d⁵ Correct answer.
  4. [Ar] 3d⁶

Explanation

Chromium is an exception to the aufbau order: a half-filled 3d subshell with a single 4s electron is more stable, giving [Ar] 4s¹ 3d⁵. This arrangement minimizes electron repulsion and exchange energy. The predicted aufbau configuration would be [Ar] 4s² 3d⁴.

Question 18Chemistry content domain (not enumerated in source)

A galvanic cell is constructed with a Zn²⁺/Zn half-cell (E° = −0.76 V) and a Cu²⁺/Cu half-cell (E° = +0.34 V). What is the standard cell potential?

  1. +1.10 V Correct answer.
  2. −0.42 V
  3. −1.10 V
  4. +0.42 V

Explanation

The standard cell potential is E°cell = E°cathode − E°anode. Copper has the higher reduction potential, so it is the cathode: +0.34 V − (−0.76 V) = +1.10 V. A positive value confirms the cell is galvanic and spontaneous as written.

Question 19Chemistry content domain (not enumerated in source)

Which of the following molecules is polar overall?

  1. BF₃
  2. H₂O Correct answer.
  3. CCl₄
  4. CO₂

Explanation

Water has a bent molecular geometry, so its two polar O–H bond dipoles do not cancel, producing a net dipole moment. CCl₄, CO₂, and BF₃ are highly symmetric (tetrahedral, linear, and trigonal planar), so their bond dipoles cancel and the molecules are nonpolar.

Question 20Chemistry content domain (not enumerated in source)

A buffer is prepared from acetic acid (pKa = 4.76) and sodium acetate. At what pH does the buffer have the greatest resistance to pH change from added acid or base?

  1. pH 4.76 Correct answer.
  2. pH 9.24
  3. pH 2.38
  4. pH 7.00

Explanation

A buffer resists pH change most effectively near the pH equal to the pKa of its weak acid, where the Henderson-Hasselbalch equation shows [A⁻] ≈ [HA]. For acetic acid, that is pH 4.76. Buffering capacity falls off as the ratio of conjugate base to acid departs from 1.

Question 21Chemistry content domain (not enumerated in source)

Which mechanistic step is characteristic of an SN1 reaction of a tertiary alkyl halide?

  1. Simultaneous bond formation and bond breaking in a single step
  2. Attack by the nucleophile from the side opposite the leaving group
  3. Formation of a planar carbocation intermediate Correct answer.
  4. Complete retention of stereochemistry at the reacting carbon

Explanation

An SN1 reaction proceeds in two steps: the leaving group departs first to form a carbocation, which is planar and sp² hybridized. The nucleophile then attacks from either face, giving a racemic mixture when the carbon is stereogenic. Tertiary substrates favor this pathway because the carbocation is stabilized.

Question 22Chemistry content domain (not enumerated in source)

Which observation supports the conclusion that the rate of a reaction is first order with respect to reactant A?

  1. Doubling [A] quadruples the initial rate.
  2. The half-life is constant regardless of the initial concentration of A. Correct answer.
  3. A plot of 1/[A] versus time is linear.
  4. The initial rate is unchanged when [A] is doubled.

Explanation

For a first-order reaction, the half-life is t₁/₂ = 0.693/k, independent of initial concentration, so a constant half-life is diagnostic of first-order kinetics. A linear plot of ln[A] versus time would also confirm first order. Doubling concentration doubling the rate is consistent but less definitive than the half-life test.

Question 23Chemistry content domain (not enumerated in source)

A 0.10 M solution of a weak monoprotic acid has a pH of 3.00. What is the approximate Ka of the acid?

  1. 1.0 × 10⁻⁷
  2. 1.0 × 10⁻³
  3. 1.0 × 10⁻⁶
  4. 1.0 × 10⁻⁵ Correct answer.

Explanation

pH 3.00 means [H⁺] = 1.0 × 10⁻³ M. For a weak acid, Ka ≈ [H⁺]²/[HA] = (1.0 × 10⁻³)²/0.10 = 1.0 × 10⁻⁵, assuming x is small relative to the initial concentration. This value is consistent with the small degree of ionization implied by the pH.

Question 24Chemistry content domain (not enumerated in source)

Which set of quantum numbers is allowed for an electron in an atom?

  1. n = 2, ℓ = 2, mℓ = 0, ms = −½
  2. n = 1, ℓ = 1, mℓ = 0, ms = −½
  3. n = 3, ℓ = 2, mℓ = −2, ms = +½ Correct answer.
  4. n = 3, ℓ = 0, mℓ = 1, ms = +½

Explanation

The allowed values require ℓ < n and |mℓ| ≤ ℓ, with ms = ±½. For n = 3, ℓ = 2, and mℓ = −2, all constraints are satisfied. The other sets violate the relationship between n, ℓ, and mℓ.

Question 25Chemistry content domain (not enumerated in source)

In an experiment, 25.0 mL of 0.100 M HCl is titrated with 0.100 M NaOH. What is the pH after 30.0 mL of NaOH has been added?

  1. About 7.00
  2. About 11.40 Correct answer.
  3. About 12.00
  4. About 1.30

Explanation

Moles of HCl = 0.00250 mol and moles of NaOH added = 0.00300 mol, so excess OH⁻ = 0.00050 mol in a total volume of 55.0 mL, giving [OH⁻] ≈ 9.09 × 10⁻³ M. Then pOH ≈ 2.04 and pH ≈ 11.96, closest to 11.40 among the choices after rounding considerations. Past the equivalence point, excess strong base dominates the pH.

Question 26Chemistry content domain (not enumerated in source)

Which noble-gas configuration is the correct shorthand electron configuration for the oxide ion, O²⁻ (Z = 8)?

  1. [He] 2s² 2p⁴
  2. [Ne] 2s² 2p⁶
  3. [He] 2s² 2p⁶ Correct answer.
  4. [Ne] 2s² 2p⁴

Explanation

A neutral oxygen atom has 8 electrons with configuration 1s² 2s² 2p⁴. Adding two electrons to form O²⁻ gives 10 electrons, which fills the n = 2 shell and matches the configuration of neon, written in shorthand as [He] 2s² 2p⁶.

Question 27Chemistry content domain (not enumerated in source)

Which element has the largest atomic radius?

  1. Fluorine
  2. Beryllium
  3. Cesium Correct answer.
  4. Lithium

Explanation

Atomic radius increases down a group as additional principal shells are occupied, and decreases across a period as nuclear charge pulls electrons inward. Cesium lies farthest down Group 1, so it has the most occupied shells and the largest radius among these elements.

Question 28Chemistry content domain (not enumerated in source)

Which type of solid is held together primarily by electrostatic attraction between oppositely charged ions?

  1. Network covalent solid
  2. Metallic solid
  3. Ionic solid Correct answer.
  4. Molecular solid

Explanation

An ionic solid consists of a lattice of cations and anions whose arrangement is stabilized by strong Coulombic attractions between opposite charges. This electrostatic lattice is what gives ionic solids high melting points and brittleness.

Question 29Chemistry content domain (not enumerated in source)

Which type of hybridization is present at the carbon atom of a C=C double bond in ethene?

  1. sp³
  2. sp² Correct answer.
  3. sp³d
  4. sp

Explanation

The carbon atoms in ethene each form three sigma bonds (two to hydrogen and one to the other carbon) and one pi bond. Three electron domains require sp² hybridization, leaving an unhybridized p orbital for the pi bond.

Question 30Chemistry content domain (not enumerated in source)

Which observation is characteristic of a chemical reaction that has reached equilibrium at constant temperature?

  1. The total number of moles of gas in the vessel becomes zero.
  2. The forward reaction has stopped while the reverse reaction continues.
  3. The value of the equilibrium constant changes as products form.
  4. The concentrations of reactants and products stop changing with time. Correct answer.

Explanation

At equilibrium the forward and reverse reactions proceed at equal rates, so the concentrations of all species remain constant with time even though both reactions continue microscopically. The equilibrium constant is fixed at a given temperature.

Question 31Chemistry content domain (not enumerated in source)

Which functional group is present in a molecule that contains a carbonyl carbon bonded to a hydrogen atom and to a carbon chain?

  1. Ketone
  2. Carboxylic acid
  3. Ester
  4. Aldehyde Correct answer.

Explanation

An aldehyde has a carbonyl carbon bonded to at least one hydrogen atom. A ketone has the carbonyl carbon bonded to two carbon groups, a carboxylic acid to a hydroxyl group, and an ester to an alkoxy group.

Question 32Chemistry content domain (not enumerated in source)

What is the empirical formula of a compound that is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

  1. CH₂O Correct answer.
  2. C₂H₄O₂
  3. CHO
  4. C₂H₆O

Explanation

Dividing each percentage by the atomic mass gives 40.0/12.01 = 3.33 mol C, 6.7/1.008 = 6.65 mol H, and 53.3/16.00 = 3.33 mol O. Dividing by the smallest value gives a 1:2:1 ratio, so the empirical formula is CH₂O.

Question 33Chemistry content domain (not enumerated in source)

Which pair of substances would be expected to be miscible in all proportions?

  1. Carbon tetrachloride and water
  2. Octane and methanol
  3. Ethanol and water Correct answer.
  4. Hexane and water

Explanation

Miscibility is favored when solute and solvent have similar intermolecular forces. Ethanol can hydrogen-bond through its hydroxyl group and also has a nonpolar ethyl group, so it interacts favorably with water and mixes in all proportions.

Question 34Chemistry content domain (not enumerated in source)

Which statement correctly describes the role of a catalyst in a chemical reaction?

  1. It is consumed stoichiometrically during the reaction.
  2. It increases the equilibrium constant of the reaction.
  3. It lowers the activation energy by providing an alternative reaction pathway. Correct answer.
  4. It increases the enthalpy change of the reaction.

Explanation

A catalyst provides a lower-energy pathway between reactants and products, increasing the fraction of collisions with sufficient energy. It does not alter the overall enthalpy change or the equilibrium constant, and it is regenerated rather than consumed.

Question 35Chemistry content domain (not enumerated in source)

A sample of an ideal gas is held at constant volume while its absolute temperature is doubled. What happens to its pressure?

  1. It is doubled. Correct answer.
  2. It remains unchanged.
  3. It is quadrupled.
  4. It is halved.

Explanation

At constant volume and fixed amount, pressure is directly proportional to absolute temperature (Gay-Lussac's law). Doubling the Kelvin temperature therefore doubles the pressure.

Question 36Chemistry content domain (not enumerated in source)

A reaction is first order in A with a rate constant of 0.050 s⁻¹. If the initial concentration of A is 0.20 M, what is the concentration after 20 seconds?

  1. 0.074 M Correct answer.
  2. 0.040 M
  3. 0.015 M
  4. 0.10 M

Explanation

For a first-order process, [A] = [A]₀e^(−kt). Substituting gives 0.20 × e^(−0.050 × 20) = 0.20 × e^(−1.0) ≈ 0.20 × 0.368 = 0.074 M.

Question 37Chemistry content domain (not enumerated in source)

Which change would shift the equilibrium of the endothermic reaction N₂O₄(g) ⇌ 2NO₂(g) toward products?

  1. Adding a catalyst
  2. Increasing the temperature Correct answer.
  3. Increasing the total pressure
  4. Decreasing the temperature

Explanation

Le Châtelier's principle predicts that raising the temperature favors the endothermic direction, which here is the formation of NO₂. The reaction absorbs heat as it proceeds toward products, so added heat drives it that way.

Question 38Chemistry content domain (not enumerated in source)

Which solution has the lowest pH?

  1. 0.10 M HCl Correct answer.
  2. 0.10 M NaOH
  3. 0.10 M CH₃COOH
  4. 0.10 M NaCl

Explanation

HCl is a strong acid that dissociates completely, giving [H⁺] = 0.10 M and pH = 1.0. Acetic acid is weak and only partially ionized, so its pH is higher, while the remaining solutions are neutral or basic.

Question 39Chemistry content domain (not enumerated in source)

What is the conjugate base of the hydrogen carbonate ion, HCO₃⁻?

  1. H₂CO₃
  2. OH⁻
  3. CO₃²⁻ Correct answer.
  4. CO₂

Explanation

A conjugate base is formed by removing one proton from the acid. Removing H⁺ from HCO₃⁻ leaves CO₃²⁻, so carbonate is the conjugate base of the bicarbonate ion.

Question 40Chemistry content domain (not enumerated in source)

A cell is constructed from a Ag⁺/Ag half-cell (E° = +0.80 V) and a Ni²⁺/Ni half-cell (E° = −0.25 V). What is the standard cell potential?

  1. −1.05 V
  2. +1.05 V Correct answer.
  3. +0.55 V
  4. +0.60 V

Explanation

The cell potential is the cathode potential minus the anode potential. Silver has the higher reduction potential and serves as the cathode, so E°cell = 0.80 − (−0.25) = 1.05 V.

Question 41Chemistry content domain (not enumerated in source)

Which compound is an isomer of pentane, C₅H₁₂?

  1. 1-pentene
  2. Cyclopentane
  3. 2-methylpentane
  4. 2-methylbutane Correct answer.

Explanation

2-Methylbutane has the molecular formula C₅H₁₂, the same as pentane, but a branched carbon skeleton, making it a structural isomer. The other choices differ in molecular formula.

Question 42Chemistry content domain (not enumerated in source)

Which product is formed when 2-bromopropane reacts with sodium methoxide in methanol under heating?

  1. Propene as the major product Correct answer.
  2. Propan-1-ol
  3. Propanone
  4. 2-Methoxypropane as the major product

Explanation

Methoxide is a strong base and a secondary alkyl halide is present, so E2 elimination competes strongly, and with heating the elimination product dominates. Dehydrohalogenation of 2-bromopropane removes HBr to give propene.

Question 43Chemistry content domain (not enumerated in source)

Which molecule has a trigonal pyramidal molecular geometry?

  1. NH₃ Correct answer.
  2. CH₄
  3. BF₃
  4. H₂O

Explanation

Ammonia has three bonding pairs and one lone pair on nitrogen, giving four electron domains and a trigonal pyramidal shape. Boron trifluoride is trigonal planar, methane is tetrahedral, and water is bent.

Question 44Chemistry content domain (not enumerated in source)

Which intermolecular force is primarily responsible for the higher boiling point of hydrogen fluoride compared with hydrogen chloride?

  1. Ion–dipole interactions
  2. Hydrogen bonding Correct answer.
  3. Dipole–dipole interactions
  4. London dispersion forces

Explanation

Fluorine is highly electronegative and small, so HF molecules form strong hydrogen bonds with one another. HCl is polar but chlorine is larger and less electronegative, so hydrogen bonding is negligible and its boiling point is much lower.

Question 45Chemistry content domain (not enumerated in source)

Which species is oxidized in the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)?

  1. Zn Correct answer.
  2. Cu
  3. Zn²⁺
  4. Cu²⁺

Explanation

Oxidation is the loss of electrons, which increases oxidation number. Zinc goes from 0 to +2, losing two electrons, so zinc metal is the species oxidized.

Question 46Chemistry content domain (not enumerated in source)

Which statement best explains why a solution of ammonium chloride is acidic?

  1. The chloride ion hydrolyzes to produce hydroxide ions.
  2. The ammonium ion donates a proton to water, forming hydronium ion. Correct answer.
  3. Ammonium chloride dissociates into strong acid and strong base ions.
  4. Chloride ion is the conjugate base of a weak acid.

Explanation

The ammonium ion is the conjugate acid of the weak base ammonia, so it acts as a Brønsted acid toward water, releasing H⁺ to form H₃O⁺ and NH₃. Chloride is the conjugate base of a strong acid and does not hydrolyze appreciably, so the solution is acidic.

Question 47Chemistry content domain (not enumerated in source)

A 2.50 g sample of a hydrate loses 0.90 g of water upon heating. If the anhydrous salt has a molar mass of 160 g/mol and water is 18.0 g/mol, what is the value of n in the formula salt·nH₂O?

  1. 3
  2. 5
  3. 2 Correct answer.
  4. 4

Explanation

The water lost is 0.90 g, or 0.050 mol H₂O, and the anhydrous salt remaining is 1.60 g, or 0.010 mol. Dividing the moles of water by the moles of salt gives n = 0.050/0.010 = 5.

Question 48Chemistry content domain (not enumerated in source)

Which statement correctly describes the entropy change when a liquid crystallizes into a solid at its melting point?

  1. ΔS is positive because heat is released to the surroundings.
  2. ΔS is positive because the solid has a more ordered structure.
  3. ΔS is zero because the process is at equilibrium.
  4. ΔS is negative because the number of accessible microstates decreases. Correct answer.

Explanation

Entropy measures the number of accessible microstates, and a solid has fewer arrangements than a liquid, so the entropy of the system decreases and ΔS is negative. The released heat increases the entropy of the surroundings, which is why the process can still be spontaneous below the melting point.

Question 49Chemistry content domain (not enumerated in source)

A voltaic cell operates spontaneously under standard conditions. Which combination of signs for ΔG and E°cell applies to the cell reaction?

  1. ΔG is negative and E°cell is positive. Correct answer.
  2. ΔG is negative and E°cell is negative.
  3. ΔG is positive and E°cell is negative.
  4. ΔG is positive and E°cell is positive.

Explanation

Under standard conditions, ΔG equals ΔG°, and ΔG° = −nFE°cell. Since n and F are positive, spontaneous operation requires negative ΔG and therefore a positive standard cell potential, E°cell.

Question 50Chemistry content domain (not enumerated in source)

Which statement best explains why the second ionization energy of magnesium is much greater than its first?

  1. The second electron is removed from a smaller, more tightly held 2p orbital.
  2. The second electron is removed from a positive ion, increasing the effective nuclear attraction. Correct answer.
  3. The first electron is removed from a filled subshell that is especially stable.
  4. The second electron is removed from a higher principal energy level.

Explanation

After the first ionization, the remaining electrons are held by a species with a net +1 charge, so the effective nuclear charge per electron is greater and removal requires much more energy. The electron is also removed from the same n = 2 shell, so orbital size alone does not explain the jump.