ACS General Chemistry Practice Test 16
60 original exam-style multiple-choice questions with answers & explanations — covering every major topic tested on the ACS General Chemistry exam.
Note: These are original practice questions written in the ACS exam style — not reproduced exam questions. Use them to test your readiness for the real ACS General Chemistry exam.
📱 How to use: Try each question first, then click “Show Answer” to check. Covers: atomic structure, bonding, stoichiometry, gases, thermochemistry, kinetics, equilibrium, acids & bases, electrochemistry and more.
⚛ Atomic Structure & Periodic Trends
Q1Magnesium has three naturally occurring isotopes: Mg-24 (78.99%), Mg-25 (10.00%), and Mg-26 (11.01%). Using isotopic masses of 23.985, 24.986, and 25.983 amu respectively, what is the average atomic mass of magnesium?
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Correct: (C)
(0.7899)(23.985) + (0.1000)(24.986) + (0.1101)(25.983) = 18.95 + 2.50 + 2.86 = 24.31 amu.
Q2What is the ground-state electron configuration of a copper atom?
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Correct: (A)
Copper is an exception to the filling order: a full 3d subshell is stabilized, so the configuration is [Ar]4s1 3d10 rather than [Ar]4s2 3d9.
Q3An element has successive ionization energies of 738, 1451, and 7733 kJ/mol. Which element is it most likely to be?
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Correct: (D)
The large jump between the second and third ionization energies shows that the third electron is removed from an inner shell, so the element has two valence electrons: magnesium (Group 2).
Q4Which ion has the largest ionic radius?
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Correct: (B)
All four ions are isoelectronic (10 electrons). With the fewest protons pulling on those electrons, O2- experiences the weakest effective nuclear charge and is the largest.
Q5Which element has the highest second ionization energy?
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Correct: (D)
Removing a second electron from Li means removing a 1s core electron from Li+, which requires far more energy than removing a valence electron from Na+, K+, or Mg+.
Q6What is the maximum number of electrons that can occupy the n = 3 energy level?
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Correct: (A)
The maximum is given by 2n^2 = 2(3)^2 = 18 electrons (3s, 3p, and 3d subshells combined).
🔬 Chemical Bonding & Molecular Geometry
Q7The best Lewis structure of which species requires the central atom to have an expanded octet?
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Correct: (C)
Phosphorus is in Period 3 and can accommodate 10 bonding electrons around it in PCl5. The other central atoms are limited to the octet (N, C, O).
Q8What is the molecular geometry of BrF3?
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Correct: (B)
BrF3 has five electron domains (three bonding pairs and two lone pairs). The lone pairs occupy equatorial positions of the trigonal bipyramidal arrangement, giving a T-shaped molecular geometry.
Q9Which molecule is nonpolar even though it contains polar bonds?
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Correct: (D)
XeF2 is linear with two polar Xe-F bonds pointing in opposite directions, so the bond dipoles cancel exactly. The other molecules are bent or pyramidal with net dipoles.
Q10Which of the following species is paramagnetic?
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Correct: (A)
Molecular orbital theory shows O2 has two unpaired electrons in its π* orbitals. N2, F2, and CO have all electrons paired and are diamagnetic.
Q11What is the hybridization of xenon in XeF2?
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Correct: (C)
Xenon has five electron domains (two bonding pairs and three lone pairs), which corresponds to sp3d hybridization and a linear molecular geometry.
Q12Which substance has the highest boiling point?
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Correct: (B)
All are nonpolar, so London dispersion forces dominate. Dispersion forces strengthen with increasing molar mass and molecular surface area, making propane the highest boiling.
Q13Which ionic compound has the largest lattice energy?
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Correct: (C)
Lattice energy increases with ion charge and decreases with ion size. MgO has 2+ and 2- ions that are both small, giving it the largest lattice energy of the set.
🧪 Stoichiometry & Aqueous Reactions
Q14What is the mass of 2.50 mol of Fe2(SO4)3?
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Correct: (A)
Molar mass = 2(55.85) + 3(32.07) + 12(16.00) = 399.91 g/mol. Mass = (2.50 mol)(399.91 g/mol) = 1.00 × 10^3 g.
Q15For the reaction N2 + 3H2 → 2NH3, what is the maximum amount (in moles) of NH3 that can form when 10.0 g of N2 reacts with 3.00 g of H2?
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Correct: (B)
Moles of N2 = 10.0 g / 28.02 g/mol = 0.357 mol, which requires 1.07 mol H2; 3.00 g H2 = 1.49 mol, so N2 is limiting. Moles of NH3 = (0.357 mol)(2/1) = 0.714 mol.
Q16Consider the reaction 2H2 + O2 → 2H2O. If 8.00 g of H2 reacts with excess O2 and 64.3 g of H2O is collected, what is the percent yield?
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Correct: (D)
Theoretical yield = (8.00 g / 2.016 g/mol)(18.02 g/mol) = 71.5 g H2O. Percent yield = (64.3 g / 71.5 g) × 100% = 90.0%.
Q17What volume of 6.00 M HCl is needed to prepare 500.0 mL of 0.250 M HCl?
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Correct: (C)
Using M1V1 = M2V2: V1 = (0.250 M)(500.0 mL) / (6.00 M) = 20.8 mL.
Q18Which pair of aqueous solutions will form a precipitate when mixed?
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Correct: (A)
Pb2+ and I- combine to form PbI2, which is insoluble. All products of the other mixtures are soluble salts, water, or gases.
Q19What volume of 0.200 M NaOH is required to neutralize 25.00 mL of 0.100 M H2SO4?
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Correct: (D)
H2SO4 provides two protons: moles of H+ = 2(0.02500 L)(0.100 M) = 0.00500 mol. Volume of NaOH = 0.00500 mol / 0.200 M = 0.0250 L = 25.00 mL.
Q20What volume of O2 gas at STP is required for the complete combustion of 5.00 g of C2H6? (C2H6 + 7/2 O2 → 2CO2 + 3H2O)
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Correct: (B)
Moles of C2H6 = 5.00 g / 30.07 g/mol = 0.1663 mol. Moles of O2 = (0.1663 mol)(7/2) = 0.582 mol. Volume at STP = (0.582 mol)(22.4 L/mol) = 13.0 L.
🌀 Gases & Intermolecular Forces
Q21A 2.50 L steel cylinder contains 0.400 mol of helium gas at 47°C. What is the pressure inside the cylinder?
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Correct: (D)
T = 320 K; P = nRT/V = (0.400)(0.08206)(320)/2.50 = 4.20 atm.
Q22A mixture of 3.0 mol CH4 and 1.0 mol CO2 exerts a total pressure of 4.0 atm. What is the partial pressure of CH4?
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Correct: (A)
Mole fraction of CH4 = 3.0/4.0 = 0.75; P(CH4) = 0.75 × 4.0 = 3.0 atm.
Q23Four gases are each at 300 K. Which has the greatest root-mean-square molecular speed?
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Correct: (D)
urms = sqrt(3RT/M); at the same temperature the lightest gas (H2, M = 2.0 g/mol) moves fastest.
Q24Which gas has the largest van der Waals constant a, which corrects for attractive forces between molecules?
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Correct: (C)
The constant a measures attraction strength; polar H2O with hydrogen bonding has far stronger attractions than the nonpolar choices.
Q25Which substance has the highest boiling point?
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Correct: (D)
All are nonpolar, so London dispersion forces dominate; these strengthen with molar mass, making n-butane the highest boiling.
🔥 Thermochemistry
Q26Consider N2(g) + 3H2(g) → 2NH3(g), ΔH = -92 kJ. How much heat is released when 4.0 mol of NH3 is formed?
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Correct: (B)
(4.0 mol/2) × 92 kJ = 184 kJ released.
Q27A 25.0 g metal sample at 95.0°C is placed in 50.0 g of water at 22.0°C. The final equilibrium temperature is 26.4°C. What is the specific heat of the metal? (c_water = 4.184 J/g·°C; assume no heat loss to surroundings)
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Correct: (B)
Heat lost by metal = heat gained by water: (25.0)(c)(68.6) = (50.0)(4.184)(4.4); c = 0.54 J/g·°C.
Q28Given: 2H2(g) + O2(g) → 2H2O(l), ΔH = -572 kJ; H2(g) + 1/2 O2(g) → H2O(g), ΔH = -242 kJ. Calculate ΔH for the vaporization of water: H2O(l) → H2O(g).
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Correct: (A)
Formation of 1 mol liquid water releases 286 kJ (half of -572); ΔHvap = -242 – (-286) = +44 kJ/mol.
Q29Calculate ΔH for 2SO2(g) + O2(g) → 2SO3(g) using ΔHf° values (kJ/mol): SO2(g) = -297, SO3(g) = -396.
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Correct: (C)
ΔH = 2(-396) – 2(-297) = -198 kJ.
Q30Estimate ΔH for N2(g) + 3H2(g) → 2NH3(g) using bond enthalpies (kJ/mol): N≡N = 941, H-H = 436, N-H = 391.
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Correct: (D)
ΔH = 941 + 3(436) – 6(391) = 941 + 1308 – 2346 = -97 kJ.
Q31Which of the following processes is endothermic?
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Correct: (C)
Photosynthesis absorbs light energy to build glucose, so ΔH > 0; the others all release heat.
⚡ Quantum Theory & Electronic Structure
Q32What is the energy of a photon with a wavelength of 450 nm? (h = 6.626 × 10^-34 J·s, c = 3.00 × 10^8 m/s)
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Correct: (B)
E = hc/λ = (6.626 × 10^-34)(3.00 × 10^8)/(450 × 10^-9) = 4.42 × 10^-19 J.
Q33What is the ground-state electron configuration of sulfur (Z = 16)?
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Correct: (A)
Sulfur has 16 electrons: 1s2 2s2 2p6 3s2 3p4 = [Ne]3s2 3p4.
Q34How many orbitals are contained in the n = 3 principal energy level?
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Correct: (C)
The number of orbitals in shell n is n^2 = 9 (one 3s, three 3p, five 3d).
Q35What is the electron configuration of the Mg2+ ion?
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Correct: (A)
Mg is [Ne]3s2; removing the two valence electrons leaves the neon core 1s2 2s2 2p6.
⏱ Chemical Kinetics
Q36For the reaction X + 2Y → products, the following initial-rate data were collected. Exp 1: [X] = 0.10 M, [Y] = 0.10 M, rate = 1.5 × 10^-3 M/s. Exp 2: [X] = 0.30 M, [Y] = 0.10 M, rate = 4.5 × 10^-3 M/s. Exp 3: [X] = 0.10 M, [Y] = 0.30 M, rate = 1.35 × 10^-2 M/s. What is the rate law?
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Correct: (D)
Tripling [X] triples the rate (1st order in X); tripling [Y] increases the rate ninefold (2nd order in Y).
Q37What are the units of the rate constant k for a first-order reaction?
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Correct: (A)
From rate (M/s) = k(M), k must have units of s^-1.
Q38A first-order reaction has k = 0.0231 min^-1. What is its half-life?
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Correct: (C)
t1/2 = 0.693/k = 0.693/0.0231 = 30 min.
Q39The activation energy of a reaction is increased while the temperature is held constant. What happens to the rate constant?
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Correct: (B)
From k = Ae^(-Ea/RT), a larger Ea at fixed T gives a smaller rate constant.
Q40A proposed mechanism is: Step 1 (slow): O3 + NO → NO2 + O2; Step 2 (fast): NO2 + O → NO + O2. What is the rate law consistent with this mechanism?
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Correct: (A)
The slow step is rate-determining, so the rate law follows its stoichiometry: rate = k[O3][NO].
Q41A plot of 1/[A] versus time for a reaction yields a straight line. What is the reaction order with respect to A?
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Correct: (B)
The second-order integrated rate law is 1/[A]t = kt + 1/[A]0, which is linear in 1/[A].
⇆ Chemical Equilibrium
Q42For the reaction CO(g) + Cl2(g) ⇌ COCl2(g), Kc = 1.2 × 10^3 at 600 K. What is Kp at this temperature?
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Correct: (B)
Δn = 1 – 2 = -1, so Kp = Kc/RT = 1200/(0.0821 × 600) ≈ 24.
Q43For the endothermic reaction N2O4(g) ⇌ 2NO2(g), ΔH° = +57 kJ/mol, which change will increase the equilibrium concentration of N2O4?
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Correct: (D)
Lowering the temperature favors the exothermic reverse reaction, increasing the equilibrium concentration of N2O4.
Q44For the reaction 2SO3(g) ⇌ 2SO2(g) + O2(g), Kc = 0.36. A mixture contains [SO3] = 0.50 M, [SO2] = 0.10 M, and [O2] = 0.10 M. What will happen?
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Correct: (C)
Q = (0.10)^2(0.10)/(0.50)^2 = 0.004, which is less than Kc, so the reaction shifts toward the products.
Q45PCl5(g) ⇌ PCl3(g) + Cl2(g) has Kc = 0.040 at 250°C. If 1.0 mol of PCl5 is placed in a 2.0-L vessel, what percent of the PCl5 dissociates at equilibrium?
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Correct: (C)
With [PCl5]0 = 0.50 M, solving x^2/(0.50 – x) = 0.040 gives x = 0.12 M, so 0.12/0.50 = 25% dissociates.
Q46The Ksp of Mg(OH)2 is 1.8 × 10^-11. What is the molar solubility of Mg(OH)2 in pure water?
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Correct: (B)
From Ksp = 4s^3, s = (1.8 × 10^-11/4)^(1/3) = 1.7 × 10^-4 M.
Q47The Ksp of CaF2 is 3.9 × 10^-11. What is the solubility of CaF2 in 0.050 M NaF?
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Correct: (D)
With the common ion, s = Ksp/[F-]^2 = 3.9 × 10^-11/(0.050)^2 = 1.6 × 10^-8 M.
🧫 Acids, Bases & Buffers
Q48What is the pH of 0.10 M methylamine, CH3NH2 (Kb = 4.4 × 10^-4)?
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Correct: (B)
[OH-] = (Kb × C)^(1/2) = 6.6 × 10^-3 M, so pOH = 2.18 and pH = 11.82.
Q49The Kb of NH3 is 1.8 × 10^-5. What is the Ka of the conjugate acid NH4+?
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Correct: (D)
Ka = Kw/Kb = 1.0 × 10^-14/1.8 × 10^-5 = 5.6 × 10^-10.
Q50A buffer solution contains acetic acid and sodium acetate. What happens to the pH when a small amount of NaOH is added?
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Correct: (D)
The added OH- is consumed by the weak acid, so the ratio changes only slightly and the pH rises only slightly.
Q51A buffer is prepared from 0.30 M NH3 and 0.20 M NH4Cl (Kb of NH3 = 1.8 × 10^-5). What is the pH of the buffer?
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Correct: (A)
pOH = pKb + log([NH4+]/[NH3]) = 4.74 + log(0.20/0.30) = 4.57, so pH = 14 – 4.57 = 9.43.
Q5220.0 mL of 0.20 M HNO3 is titrated with 0.20 M KOH. What is the pH after 10.0 mL of KOH has been added?
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Correct: (D)
Excess H+ = 0.0040 – 0.0020 = 0.0020 mol in 30.0 mL gives [H+] = 0.067 M, so pH = 1.18.
Q53Which of the following solutions has the greatest buffer capacity?
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Correct: (A)
Buffer capacity increases with the total concentration of the conjugate acid-base pair.
🔋 Thermodynamics & Electrochemistry
Q54Which process shows the greatest increase in entropy?
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Correct: (C)
Sublimation converts a solid directly to a gas, the largest increase in disorder among the choices.
Q55A reaction has ΔH° = +58 kJ/mol and ΔS° = +176 J/mol·K. What are ΔG° and spontaneity at 500 K?
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Correct: (B)
ΔG° = 58000 – 500(176) = -30000 J/mol = -30 kJ/mol, so the reaction is spontaneous.
Q56Consider the cell Pt | H2(1 atm) | H+(1 M) || Cu2+(1 M) | Cu. Given E°(Cu2+/Cu) = +0.34 V, which statement is correct?
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Correct: (A)
E°cell = 0.34 – 0.00 = 0.34 V; hydrogen is oxidized, so the hydrogen electrode is the anode.
Q57How long must a current of 3.00 A flow to deposit 5.00 g of copper from a Cu2+ solution?
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Correct: (B)
5.00 g Cu = 0.0787 mol Cu requires 0.157 mol e- = 15183 C, so t = 15183/3.00 = 5061 s ≈ 84 min.
Q58Given the standard reduction potentials F2/F- (+2.87 V), Cl2/Cl- (+1.36 V), Br2/Br- (+1.09 V), I2/I- (+0.54 V), which species is the strongest oxidizing agent?
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Correct: (A)
The species with the largest positive reduction potential, F2, is the strongest oxidizing agent.
☢ Nuclear & Descriptive Chemistry
Q59Which statement correctly describes nuclear fission?
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Correct: (A)
Fission is the splitting of a heavy nucleus into lighter fragments with the release of neutrons and energy.
Q60Which element has the smallest atomic radius?
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Correct: (B)
Atomic radius decreases across a period and increases down a group. Mg is farthest to the right and highest among these choices, making it the smallest.
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