ACS General Chemistry Practice Test 1
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
Q1The ground-state electron configuration of copper (Z = 29) is
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Correct: (B)
A half-filled 4s and a completely filled 3d subshell is more stable, so one 4s electron is promoted to give [Ar] 4s^1 3d^10.
Q2Which of the following elements has the highest first ionization energy?
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Correct: (D)
Ionization energy increases across a period with increasing effective nuclear charge; Cl is farthest right of the four.
Q3Which list orders the atoms by increasing atomic radius?
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Correct: (A)
Atomic radius decreases across a period (F < O) and increases down a group (O < S, F < Cl), giving F < O < Cl < S.
Q4Chlorine has two isotopes: ^35Cl (34.969 u, 75.78% abundance) and ^37Cl (36.966 u, 24.22% abundance). The average atomic mass of chlorine is closest to
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Correct: (C)
(34.969 × 0.7578) + (36.966 × 0.2422) = 26.50 + 8.95 = 35.45 u.
Q5Which atom has the most exothermic (largest magnitude) electron affinity?
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Correct: (A)
Electron affinity becomes more exothermic across a period; F (328 kJ/mol) exceeds O, N is slightly endothermic, and Ne does not form a stable anion.
Q6The photoelectron spectrum of a gaseous element shows three peaks with relative intensities 2 : 2 : 4, corresponding to the 1s, 2s, and 2p subshells. The element is
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Correct: (D)
Intensities 2 : 2 : 4 correspond to 1s^2 2s^2 2p^4, which is the configuration of oxygen.
🔬 Chemical Bonding & Molecular Geometry
Q7In the best Lewis structure of the thiocyanate ion, NCS^-, the formal charge on the carbon atom is
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Correct: (C)
In [N≡C−S]^-, carbon forms four bonds and has no lone pairs: 4 − (8/2) = 0.
Q8The molecular geometry of XeF4 is
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Correct: (B)
Xe has six electron domains (4 bonding pairs + 2 lone pairs); the lone pairs occupy axial positions, leaving a square planar shape.
Q9Which molecule has the smallest bond angle?
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Correct: (C)
H2O is bent with an angle of about 104.5°, smaller than NH3 (107°), CH4 (109.5°), and BF3 (120°).
Q10The hybridization of the central sulfur atom in SF4 is
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Correct: (A)
SF4 has five electron domains (4 bonds + 1 lone pair), requiring five hybrid orbitals: sp^3d.
Q11How many sigma (σ) and pi (π) bonds are present in one molecule of HCN?
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Correct: (D)
The H−C single bond contributes 1 σ, and the C≡N triple bond contributes 1 σ + 2 π, for a total of 2 σ and 2 π.
Q12Which molecule has a dipole moment of zero?
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Correct: (B)
CO2 is linear, so the two equal and opposite C=O bond dipoles cancel.
Q13Which compound has the highest lattice energy?
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Correct: (D)
Lattice energy increases with ionic charge; MgO contains Mg^2+ and O^2-, giving far stronger attraction than the +1/−1 salts.
🧪 Stoichiometry & Aqueous Reactions
Q14How many atoms are in 12.0 g of magnesium? (molar mass Mg = 24.3 g/mol; Avogadro's number = 6.022 × 10^23)
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Correct: (A)
12.0 g / 24.3 g/mol = 0.4938 mol; 0.4938 × 6.022 × 10^23 = 2.97 × 10^23 atoms.
Q158.0 g of H2 (2.02 g/mol) reacts with 32.0 g of O2 (32.00 g/mol) according to 2H2 + O2 → 2H2O. What is the maximum mass of water (18.02 g/mol) that can form?
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Correct: (C)
8.0 g H2 = 3.96 mol H2 needs 1.98 mol O2, but only 1.00 mol O2 is present, so O2 limits: 1.00 mol O2 → 2.00 mol H2O = 36.0 g.
Q1625.0 g of CaCO3 (100.1 g/mol) is heated: CaCO3 → CaO + CO2. If 11.2 g of CaO (56.08 g/mol) is obtained, the percent yield is
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Correct: (B)
Theoretical CaO = (25.0 / 100.1) × 56.08 = 14.0 g; (11.2 / 14.0) × 100% = 80.0%.
Q17What volume of 6.00 M HCl is needed to prepare 500.0 mL of 0.250 M HCl?
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Correct: (D)
M1V1 = M2V2 gives V1 = (0.250 × 500.0) / 6.00 = 20.8 mL.
Q18The net ionic equation for the reaction of aqueous silver nitrate with aqueous sodium chloride is
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Correct: (A)
AgCl precipitates while Na^+ and NO3^- are spectator ions and cancel.
Q19When the reaction Cr2O7^2- + Fe^2+ + H^+ → Cr^3+ + Fe^3+ + H2O is balanced in acidic solution, the coefficient of Fe^2+ is
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Correct: (C)
Each Cr goes from +6 to +3 (2 Cr gain 6 e^- total); each Fe^2+ loses 1 e^-, so 6 Fe^2+ are required: Cr2O7^2- + 6Fe^2+ + 14H^+ → 2Cr^3+ + 6Fe^3+ + 7H2O.
Q20A compound contains 40.0% C, 6.7% H, and 53.3% O by mass. Its empirical formula is
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Correct: (B)
Moles per 100 g: C = 3.33, H = 6.65, O = 3.33; dividing by 3.33 gives a 1 : 2 : 1 ratio, CH2O.
🌀 Gases & Intermolecular Forces
Q21A 2.50 L flask contains 0.400 mol of N2 gas at 27°C. What is the pressure of the gas in the flask? (R = 0.0821 L·atm/mol·K)
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Correct: (B)
P = nRT/V = (0.400)(0.0821)(300)/(2.50) = 3.94 atm.
Q22An unknown gas X effuses through a pinhole 2.00 times faster than CO2 (molar mass 44.0 g/mol) under identical conditions. What is the molar mass of X?
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Correct: (D)
By Graham's law, (rate X/rate CO2)^2 = M(CO2)/M(X), so 4.00 = 44.0/M(X), giving M(X) = 11.0 g/mol.
Q23A 5.00 L vessel at 25°C contains a mixture of He and O2. The partial pressure of He is 2.00 atm and the total pressure is 3.50 atm. How many moles of O2 are present? (R = 0.0821 L·atm/mol·K)
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Correct: (A)
P(O2) = 3.50 − 2.00 = 1.50 atm; n = PV/RT = (1.50)(5.00)/(0.0821)(298) = 0.307 mol.
Q24Which of the following substances has the highest boiling point?
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Correct: (C)
CH3OH exhibits hydrogen bonding, the strongest intermolecular force among the choices, giving it the highest boiling point.
Q25Under which conditions does a real gas deviate most from ideal gas behavior?
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Correct: (B)
Low temperature reduces kinetic energy so attractions matter, and high pressure forces molecules close together, increasing the effect of molecular volume.
🔥 Thermochemistry
Q26How much heat is required to raise the temperature of 150.0 g of water from 25.0°C to 80.0°C? (specific heat of water = 4.18 J/g·°C)
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Correct: (D)
q = mcΔT = (150.0)(4.18)(55.0) = 3.45 × 10^4 J = 34.5 kJ.
Q27Given the following enthalpies:
C(s) + O2(g) → CO2(g) ΔH = −393.5 kJ
H2(g) + 1/2 O2(g) → H2O(l) ΔH = −285.8 kJ
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ΔH = −890.4 kJ
What is the standard enthalpy of formation of CH4(g)?
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Correct: (C)
By Hess's law, ΔHf = (−393.5) + 2(−285.8) − (−890.4) = −74.7 kJ/mol.
Q28Use average bond enthalpies to estimate ΔH for the reaction N2(g) + 3 H2(g) → 2 NH3(g). (N≡N = 941 kJ/mol, H−H = 436 kJ/mol, N−H = 391 kJ/mol)
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Correct: (A)
Bonds broken: 941 + 3(436) = 2249 kJ; bonds formed: 6(391) = 2346 kJ; ΔH = 2249 − 2346 = −97 kJ.
Q29A 50.0 g sample of an unknown metal at 100.0°C is placed in 100.0 g of water at 20.0°C. The final equilibrium temperature is 24.6°C. What is the specific heat of the metal? (specific heat of water = 4.18 J/g·°C)
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Correct: (B)
Heat gained by water = (100.0)(4.18)(4.6) = 1923 J; c(metal) = 1923/[(50.0)(75.4)] = 0.510 J/g·°C.
Q30Which of the following processes is exothermic?
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Correct: (D)
Combustion releases heat to the surroundings (ΔH < 0); the others all absorb heat.
Q31Calculate ΔH for the reaction 2 SO2(g) + O2(g) → 2 SO3(g) given ΔHf°(SO2) = −296.8 kJ/mol and ΔHf°(SO3) = −395.7 kJ/mol.
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Correct: (A)
ΔH = 2(−395.7) − [2(−296.8) + 0] = −197.8 kJ.
⚡ Quantum Theory & Electronic Structure
Q32What is the energy of a single photon of ultraviolet light with a wavelength of 250 nm? (h = 6.626 × 10^-34 J·s, c = 3.00 × 10^8 m/s)
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Correct: (C)
E = hc/λ = (6.626 × 10^-34)(3.00 × 10^8)/(2.50 × 10^-7) = 7.95 × 10^-19 J.
Q33Which of the following sets of quantum numbers (n, l, ml, ms) is allowed for an electron in an atom?
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Correct: (B)
l must be less than n and |ml| ≤ l; only set B satisfies all rules.
Q34What is the de Broglie wavelength of an electron traveling at 2.19 × 10^6 m/s? (h = 6.626 × 10^-34 J·s, mass of electron = 9.11 × 10^-31 kg)
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Correct: (D)
λ = h/mv = (6.626 × 10^-34)/(9.11 × 10^-31)(2.19 × 10^6) = 3.32 × 10^-10 m.
Q35An electron in a hydrogen atom falls from the n = 3 level to the n = 2 level. What is the energy of the photon emitted? (E = −13.6 eV/n^2)
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Correct: (A)
ΔE = 13.6(1/2^2 − 1/3^2) = 13.6(5/36) = 1.89 eV.
⏱ Chemical Kinetics
Q36For the reaction A + B → products, the following initial-rate data were collected.
Experiment 1: [A] = 0.10 M, [B] = 0.10 M, initial rate = 2.0 × 10^-3 M/s
Experiment 2: [A] = 0.20 M, [B] = 0.10 M, initial rate = 8.0 × 10^-3 M/s
Experiment 3: [A] = 0.10 M, [B] = 0.20 M, initial rate = 2.0 × 10^-3 M/s
What is the rate law for this reaction?
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Correct: (C)
Doubling [A] quadruples the rate (2nd order in A); doubling [B] leaves the rate unchanged (zero order in B).
Q37The decomposition of N2O5 is first order with k = 0.0693 min^-1. How long will it take for 75% of a sample of N2O5 to be consumed?
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Correct: (A)
t1/2 = 0.693/0.0693 = 10 min; 75% consumed leaves 1/4 remaining, which requires 2 half-lives = 20 min.
Q38A proposed mechanism for 2 NO(g) + O2(g) → 2 NO2(g) is:
Step 1: NO + NO → N2O2 (slow)
Step 2: N2O2 + O2 → 2 NO2 (fast)
What rate law is predicted by this mechanism?
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Correct: (D)
The rate law comes from the slow (rate-determining) step: Rate = k[NO][NO] = k[NO]^2.
Q39A reaction has k = 1.0 × 10^-3 s^-1 at 300 K and an activation energy of 50 kJ/mol. What is the approximate value of k at 310 K? (R = 8.314 J/mol·K)
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Correct: (B)
ln(k2/k1) = (50,000/8.314)(1/300 − 1/310) = 0.647, so k2/k1 = e^0.647 = 1.91; k2 ≈ 1.9 × 10^-3 s^-1.
Q40On a reaction coordinate diagram, the reactants lie at 40 kJ/mol, the transition state at 110 kJ/mol, and the products at 90 kJ/mol. What are the activation energy (Ea) and ΔH for the forward reaction?
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Correct: (B)
Ea = 110 − 40 = 70 kJ/mol; ΔH = 90 − 40 = +50 kJ/mol (endothermic).
Q41The decomposition of HI(g) is second order with k = 2.4 × 10^-2 M^-1·s^-1 at 500 K. If the initial [HI] is 0.80 M, how long will it take for [HI] to fall to 0.20 M?
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Correct: (A)
1/[A] = kt + 1/[A]0 gives t = (1/0.20 − 1/0.80)/0.024 = 3.75/0.024 = 156 s ≈ 1.6 × 10^2 s.
⇆ Chemical Equilibrium
Q42For the reaction 2 SO2(g) + O2(g) ⇌ 2 SO3(g), what is the correct expression for Kc?
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Correct: (D)
Kc = (products)^coefficients/(reactants)^coefficients = [SO3]^2/([SO2]^2[O2]).
Q43For N2(g) + 3 H2(g) ⇌ 2 NH3(g) at a certain temperature, the equilibrium concentrations are [N2] = 0.20 M, [H2] = 0.60 M, and [NH3] = 0.30 M. What is the value of Kc?
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Correct: (C)
Kc = [NH3]^2/([N2][H2]^3) = (0.30)^2/(0.20 × 0.60^3) = 0.09/0.0432 = 2.1.
Q44For N2O4(g) ⇌ 2 NO2(g), Kc = 4.6 × 10^-3 at 298 K. What is Kp at this temperature? (R = 0.0821 L·atm/mol·K)
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Correct: (A)
Δn = 2 − 1 = 1, so Kp = Kc(RT)^1 = (4.6 × 10^-3)(0.0821)(298) = 0.11.
Q45Consider CO(g) + 2 H2(g) ⇌ CH3OH(g), ΔH° = −91 kJ/mol. Which change will increase the equilibrium yield of CH3OH?
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Correct: (D)
The reaction is exothermic and goes from 3 moles of gas to 1; higher pressure shifts equilibrium toward fewer moles of gas, producing more CH3OH.
Q46For H2(g) + I2(g) ⇌ 2 HI(g), Kc = 50 at 448°C. A flask initially contains 0.50 M H2 and 0.50 M I2 with no HI. What is [HI] at equilibrium?
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Correct: (B)
(2x)^2/(0.50 − x)^2 = 50 gives 2x/(0.50 − x) = 7.07, so x = 0.390 and [HI] = 2x = 0.78 M.
Q47For 2 NO2(g) ⇌ N2O4(g), Kc = 170 at 298 K. A flask contains [NO2] = 0.10 M and [N2O4] = 0.50 M. Which statement is correct?
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Correct: (C)
Q = [N2O4]/[NO2]^2 = 0.50/(0.10)^2 = 50; since Q < K, the reaction proceeds toward products.
🧫 Acids, Bases & Buffers
Q48Equal volumes of 0.020 M HCl and 0.020 M HNO3 are mixed. Assuming volumes are additive and both acids dissociate completely, what is the pH of the resulting solution?
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Correct: (B)
Each acid is diluted by half on mixing, so [H+] = 0.010 + 0.010 = 0.020 M; pH = −log(2.0×10^-2) = 1.70.
Q49Which of the following aqueous mixtures acts as a buffer?
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Correct: (D)
It contains a weak base and its conjugate acid in comparable amounts; the others lack a conjugate acid-base pair.
Q50A 0.20 M solution of the weak acid HA has Ka = 4.0×10^-5. What is the pH of the solution?
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Correct: (A)
[H+] = sqrt(Ka·C) = sqrt(8.0×10^-6) = 2.8×10^-3 M, so pH = 2.55.
Q51For the acid H2PO4^-, Ka = 6.2×10^-8. What is Kb for its conjugate base, HPO4^2-? (Kw = 1.0×10^-14)
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Correct: (C)
Kb = Kw/Ka = (1.0×10^-14)/(6.2×10^-8) = 1.6×10^-7.
Q5225.0 mL of 0.100 M CH3COOH (Ka = 1.8×10^-5) is titrated with 0.100 M NaOH. What is the pH at the equivalence point?
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Correct: (D)
At equivalence only CH3COO^- remains, and it hydrolyzes to produce OH^-, so the solution is basic.
Q53A buffer contains 0.30 M CH3COO^- and 0.10 M CH3COOH. If pKa = 4.75 for CH3COOH, what is the pH of the buffer?
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Correct: (B)
pH = 4.75 + log(0.30/0.10) = 4.75 + 0.48 = 5.23.
🔋 Thermodynamics & Electrochemistry
Q54What is the sign of ΔS° for the reaction 2 H2(g) + O2(g) → 2 H2O(l)?
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Correct: (A)
Three moles of gas form a liquid with no gaseous product, so disorder decreases.
Q55A reaction has ΔH° = −120 kJ/mol and ΔS° = −0.300 kJ/(mol·K). At which temperatures is the reaction spontaneous?
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Correct: (D)
ΔG = 0 at T = ΔH/ΔS = 400 K; with ΔH < 0 and ΔS < 0, ΔG < 0 only below 400 K.
Q56At 298 K, a reaction has equilibrium constant K = 1.0×10^5. What is ΔG°? (R = 8.314 J/(mol·K))
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Correct: (C)
ΔG° = −RT ln K = −(8.314)(298)(11.51) = −2.85×10^4 J/mol = −28.5 kJ/mol.
Q57Given: Zn^2+ + 2e^- → Zn, E° = −0.76 V; Cu^2+ + 2e^- → Cu, E° = +0.34 V. For the galvanic cell Zn|Zn^2+||Cu^2+|Cu, which statement is true?
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Correct: (B)
E°cell = E°cathode − E°anode = 0.34 − (−0.76) = 1.10 V; zinc is the anode and is oxidized.
Q58How many grams of copper are deposited when 0.500 mol of electrons passes through a Cu^2+ solution? (Cu^2+ + 2e^- → Cu; molar mass of Cu = 63.55 g/mol)
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Correct: (A)
Moles Cu = 0.500/2 = 0.250 mol; mass = 0.250 × 63.55 = 15.9 g.
☢ Nuclear & Descriptive Chemistry
Q59In the nuclear decay ^238U → ^234Th + ?, what is the missing particle?
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Correct: (C)
The mass number drops by 4 and the atomic number by 2, matching ^4He, an alpha particle.
Q60An 80.0 g sample of a radioisotope with a half-life of 12 days decays for 36 days. What mass remains?
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Correct: (B)
36/12 = 3 half-lives, so 80.0 × (1/2)^3 = 10.0 g remains.
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