ACS General Chemistry Practice Test 9
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
Q1A naturally occurring sample of gallium contains 60.1% Ga-69 (atomic mass 68.926 u) and the remainder Ga-71 (atomic mass 70.925 u). What is the average atomic mass of gallium in this sample?
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Correct: (B)
Weighted average = 0.601(68.926) + 0.399(70.925) = 41.42 + 28.30 = 69.73 u.
Q2Which of the following electron configurations represents a ground-state chromium atom?
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Correct: (B)
Chromium is an exception to the Aufbau filling order: a half-filled 4s and half-filled 3d subshell is more stable, giving [Ar] 4s1 3d5 rather than [Ar] 4s2 3d4.
Q3For the ion Se2−, which of the following gives the correct ground-state electron configuration?
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Correct: (B)
Neutral Se is [Ar] 4s2 3d10 4p4. Gaining 2 electrons fills the 4p subshell, giving [Ar] 4s2 3d10 4p6 (isoelectronic with Kr).
Q4Which of the following lists the elements in order of increasing first ionization energy?
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Correct: (A)
Ionization energy increases across a period (left to right) as effective nuclear charge rises, so Al < Si < S < Cl.
Q5Which of the following ions is largest?
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Correct: (D)
All four are isoelectronic (10 electrons), so size is set by nuclear charge: the fewest protons (O2−, Z = 8) exert the weakest pull, making O2− the largest.
Q6An element has the following successive ionization energies (kJ/mol): IE1 = 737, IE2 = 1451, IE3 = 7733. Which element is most likely being described?
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Correct: (B)
The huge jump between IE2 and IE3 indicates removal of a core electron after two valence electrons are removed, identifying a Group 2A element. Of the choices, Mg (Z = 12) fits; Na and K are Group 1A (jump after IE1) and Al is Group 3A (jump after IE3).
🔬 Chemical Bonding & Molecular Geometry
Q7What is the formal charge on the central atom in the Lewis structure of the carbonate ion, CO3 2−, drawn with the maximum number of nonbonding electrons satisfying octets?
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Correct: (C)
Carbon has 4 valence electrons, forms 4 bonds total (one double + two single in any resonance form) and has no lone pairs: 4 − (0 + 4) = 0.
Q8How many sigma (σ) bonds and pi (π) bonds are present in a molecule of formic acid, HCOOH?
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Correct: (A)
The structure H−C(=O)−O−H contains four σ bonds (H−C, C=O, C−O, O−H) and one π bond (in the C=O double bond).
Q9According to VSEPR theory, what is the molecular geometry of the chlorite ion, ClO2−?
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Correct: (B)
ClO2− has 20 valence electrons; the central Cl has 2 bonding pairs and 2 lone pairs (AX2E2), giving a bent molecular geometry.
Q10Which molecule has a bond angle closest to 120°?
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Correct: (B)
BF3 is trigonal planar (AX3, no lone pairs) with ideal 120° bond angles. CH4 is 109.5°, and H2O/NH3 are near tetrahedral but compressed below 109.5° by lone pairs.
Q11What is the hybridization of the central atom in sulfur hexafluoride, SF6?
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Correct: (C)
SF6 has six bonding pairs and no lone pairs (octahedral, AX6), requiring six hybrid orbitals: sp3d2.
Q12Which of the following molecules is polar?
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Correct: (C)
SO2 is bent (AX2E), so its bond dipoles do not cancel. CCl4, CO2, and BCl3 are symmetric and nonpolar.
Q13Rank the following nitrogen-containing species in order of increasing N−O or N−N bond length: NO+, NO, NO−.
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Correct: (A)
Bond orders: NO+ = 3, NO = 2.5, NO− = 2. Higher bond order means shorter, stronger bonds, so NO+ (shortest) < NO < NO− (longest).
🧪 Stoichiometry & Aqueous Reactions
Q14Ammonium sulfate fertilizer is produced by the reaction 2NH3(g) + H2SO4(aq) → (NH4)2SO4(aq). What mass of ammonium sulfate can be produced from 34.0 g of NH3 reacting with excess sulfuric acid? (Molar masses: NH3 = 17.03 g/mol, (NH4)2SO4 = 132.15 g/mol)
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Correct: (C)
34.0 g NH3 = 1.996 mol; the 2:1 mole ratio gives 0.998 mol (NH4)2SO4, and 0.998 mol × 132.15 g/mol = 132 g.
Q15Aluminum metal is produced from aluminum chloride by the reaction 3Mg(s) + 2AlCl3(aq) → 3MgCl2(aq) + 2Al(s). If 12.2 g of Mg and 33.4 g of AlCl3 react, which reactant limits and what mass of Al forms? (Molar masses: Mg = 24.31, AlCl3 = 133.33, Al = 26.98 g/mol)
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Correct: (A)
12.2 g Mg = 0.502 mol requires 0.335 mol AlCl3, but only 0.251 mol AlCl3 is present, so AlCl3 limits; 0.251 mol Al × 26.98 g/mol = 6.76 g Al.
Q16Ammonia is synthesized by N2(g) + 3H2(g) → 2NH3(g). Starting with 50.0 g of N2 (excess H2), the reaction produces 45.0 g of NH3. What is the percent yield? (Molar masses: N2 = 28.02, NH3 = 17.03 g/mol)
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Correct: (D)
Theoretical yield: 50.0 g N2 = 1.784 mol → 3.569 mol NH3 = 60.8 g; percent yield = (45.0 / 60.8) × 100% = 74.0%.
Q17A phosphorus oxide contains 43.7% P and 56.3% O by mass. What is its empirical formula? (Molar masses: P = 30.97, O = 16.00 g/mol)
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Correct: (B)
43.7/30.97 = 1.411 mol P and 56.3/16.00 = 3.519 mol O gives a 1:2.5 ratio, which doubles to P2O5; P4O10 is the molecular formula, not the empirical formula.
Q18A chemist mixes 100. mL of 0.250 M NaCl with 200. mL of 0.100 M NaCl. What is the molarity of NaCl in the resulting solution?
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Correct: (C)
Total moles = (0.100 L)(0.250 M) + (0.200 L)(0.100 M) = 0.0450 mol in 0.300 L, so M = 0.0450/0.300 = 0.150 M.
Q19Aqueous solutions of sodium phosphate and calcium chloride are mixed: 2Na3PO4(aq) + 3CaCl2(aq) → Ca3(PO4)2(s) + 6NaCl(aq). What is the net ionic equation?
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Correct: (B)
Ca3(PO4)2 is the insoluble precipitate; Na^+ and Cl^- are spectator ions and cancel, leaving only Ca^2+ and PO4^3- in the net ionic equation.
Q20What volume of 0.250 M NaOH is required to neutralize 25.00 mL of 0.200 M H2SO4? (H2SO4 + 2NaOH → Na2SO4 + 2H2O)
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Correct: (D)
Moles H^+ = 2(0.02500 L)(0.200 M) = 0.0100 mol, requiring 0.0100 mol NaOH; V = 0.0100 mol / 0.250 M = 0.0400 L = 40.0 mL.
🌀 Gases & Intermolecular Forces
Q21What volume is occupied by 2.50 mol of N2 gas at 350 K and 1.50 atm? (R = 0.08206 L·atm/mol·K)
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Correct: (A)
V = nRT/P = (2.50)(0.08206)(350)/1.50 = 47.9 L.
Q22A 3.00 L flask contains 4.20 g of an unknown gas at 300 K and 0.980 atm. What is the molar mass of the gas? (R = 0.08206 L·atm/mol·K)
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Correct: (C)
Gas density = 4.20 g/3.00 L = 1.40 g/L; M = dRT/P = (1.40)(0.08206)(300)/0.980 = 35.2 g/mol.
Q23Hydrogen gas is collected over water at 25°C. The total pressure is 755 torr and the vapor pressure of water at 25°C is 23.8 torr. What is the partial pressure of the dry H2?
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Correct: (B)
By Dalton's law, P(H2) = P(total) − P(H2O) = 755 − 23.8 = 731 torr.
Q24A sample of methane (CH4, M = 16.04 g/mol) effuses through a pinhole in 45.0 s. How long will it take an equal amount of sulfur dioxide (SO2, M = 64.07 g/mol) to effuse under identical conditions?
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Correct: (A)
By Graham's law, t(SO2) = t(CH4) × √(M(SO2)/M(CH4)) = 45.0 × √(64.07/16.04) = 45.0 × 2.00 = 90.0 s.
Q25Which liquid has the lowest vapor pressure at 25°C?
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Correct: (D)
Ethylene glycol has two –OH groups and forms the most extensive hydrogen-bonding network, giving it the strongest intermolecular forces and therefore the lowest vapor pressure.
🔥 Thermochemistry
Q26A 45.0 g block of aluminum (specific heat capacity = 0.897 J/g·°C), initially at 95.0 °C, is dropped into a calorimeter containing 120.0 g of water at 22.0 °C. Assuming no heat is lost to the calorimeter itself, what is the final equilibrium temperature?
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Correct: (B)
Heat lost by Al = heat gained by water: 45.0(0.897)(95.0 − Tf) = 120.0(4.184)(Tf − 22.0). Solving: 3835.7 − 40.37Tf = 502.08Tf − 11045.8 → Tf = 14881.5/542.45 = 27.4 °C.
Q27Use Hess's law with the reactions below to find ΔH for 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l).
N2(g) + 3H2(g) → 2NH3(g) ΔH = −92.2 kJ
N2(g) + O2(g) → 2NO(g) ΔH = +180.5 kJ
2H2(g) + O2(g) → 2H2O(l) ΔH = −571.6 kJ
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Correct: (A)
Reverse the first (×2): +184.4 kJ; double the second: +361.0 kJ; triple the third: −1714.8 kJ. The N2 and H2 cancel, giving 4NH3 + 5O2 → 4NO + 6H2O(l) with ΔH = 184.4 + 361.0 − 1714.8 = −1169.4 kJ.
Q28Estimate ΔH for the hydrogenation of propene: C3H6(g) + H2(g) → C3H8(g), using these average bond enthalpies: C=C 614, C–C 347, C–H 413, H–H 436 kJ/mol.
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Correct: (C)
Bonds broken: 1 C=C + 6 C–H + 1 H–H = 614 + 2478 + 436 = 3528 kJ. Bonds formed: 1 C–C + 8 C–H = 347 + 3304 = 3651 kJ. ΔH = broken − formed = 3528 − 3651 = −123 kJ.
Q29Given standard enthalpies of formation: ΔHf° [CH4(g)] = −74.8, [CO2(g)] = −393.5, [H2O(l)] = −285.8 kJ/mol, calculate ΔH°rxn for CH4(g) + 2O2(g) → CO2(g) + 2H2O(l).
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Correct: (A)
ΔH° = [−393.5 + 2(−285.8)] − [−74.8 + 0] = −965.1 + 74.8 = −890.3 kJ.
Q30How much heat must be absorbed to completely melt 25.0 g of ice at 0 °C to liquid water at 0 °C? (ΔHfus of water = 6.02 kJ/mol; molar mass H2O = 18.015 g/mol)
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Correct: (D)
Moles of ice = 25.0/18.015 = 1.388 mol. q = nΔHfus = 1.388(6.02) = 8.35 kJ absorbed.
Q31Which statement correctly describes enthalpy sign conventions?
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Correct: (C)
By convention, ΔH = H(products) − H(reactants). Releasing heat makes H(products) < H(reactants), so ΔH is negative for exothermic processes.
⚡ Quantum Theory & Electronic Structure
Q32A blue-violet photon has a wavelength (λ) of 455 nm. What is the energy of one such photon? (h = 6.626 x 10^-34 J·s, c = 2.998 x 10^8 m/s)
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Correct: (A)
E = hc/λ = (6.626 x 10^-34)(2.998 x 10^8)/(455 x 10^-9) = 4.37 x 10^-19 J.
Q33A metal has a threshold frequency (ν0) of 1.10 x 10^15 Hz. Light of wavelength 220 nm strikes the metal surface. What happens?
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Correct: (B)
Photon energy = hc/λ = 9.03 x 10^-19 J. Work function = hν0 = 7.29 x 10^-19 J. Since E(photon) > work function, electrons are ejected with KE = 9.03 x 10^-19 − 7.29 x 10^-19 = 1.74 x 10^-19 J.
Q34Which of the following sets of quantum numbers (n, l, ml, ms) is NOT permitted for an electron in an atom?
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Correct: (C)
For n = 2, the allowed values of l are 0 and 1 (l must be less than n). l = 2 is not allowed, so the entire set is forbidden.
Q35An electron travels at 2.19 x 10^6 m/s (the speed of the electron in the first Bohr orbit). What is its de Broglie wavelength? (h = 6.626 x 10^-34 J·s, me = 9.11 x 10^-31 kg)
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Correct: (D)
λ = h/mv = 6.626 x 10^-34 / (9.11 x 10^-31)(2.19 x 10^6) = 3.32 x 10^-10 m.
⏱ Chemical Kinetics
Q36Consider the reaction 2X(g) + Y(g) → Z(g). Three initial-rate experiments gave:
Exp 1: [X] = 0.10 M, [Y] = 0.10 M, rate = 4.0×10−4 M/s
Exp 2: [X] = 0.20 M, [Y] = 0.10 M, rate = 1.6×10−3 M/s
Exp 3: [X] = 0.10 M, [Y] = 0.20 M, rate = 8.0×10−4 M/s
What is the rate law?
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Correct: (C)
Doubling [X] (Exp 1 → 2) quadruples the rate, so the reaction is 2nd order in X; doubling [Y] (Exp 1 → 3) doubles the rate, so it is 1st order in Y.
Q37The first-order decomposition N2O5(g) → products has a half-life of 24.0 min at a certain temperature. A sample starts at [N2O5] = 0.80 M. What is [N2O5] after 72.0 min?
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Correct: (B)
72.0 min is exactly 3 half-lives: 0.80 → 0.40 → 0.20 → 0.10 M.
Q38For the gas-phase reaction 2HI(g) → H2(g) + I2(g), the rate constant is 2.5×10−3 s−1 at 25°C and 2.0×10−2 s−1 at 45°C. What is the activation energy?
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Correct: (B)
From ln(k2/k1) = (Ea/R)(1/T1 − 1/T2): ln(8.0) = 2.08 = (Ea/8.314)(1/298 − 1/318), giving Ea ≈ 8.2×10⁴ J/mol ≈ 82 kJ/mol.
Q39For the reaction 2NO(g) + Cl2(g) → 2NOCl(g), the experimentally determined rate law is rate = k[NO]²[Cl2]. What are the units of the rate constant k?
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Correct: (D)
The reaction is 3rd order overall, so units of k are M^(1−3)·s−1 = M−2·s−1, which makes rate come out in M/s.
Q40A catalyst is added to the reaction 2SO2(g) + O2(g) → 2SO3(g). Which statement correctly describes the effect of the catalyst?
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Correct: (A)
A catalyst speeds the reaction by offering a lower-Ea mechanism; it changes neither K nor ΔH and is regenerated.
Q41The dimerization 2A(g) → A2(g) is second order in A, with k = 0.050 L·mol−1·s−1 at 500 K. If [A]0 = 0.20 M, what is [A] after 50.0 s?
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Correct: (C)
Second-order integrated law: 1/[A] = 1/0.20 + (0.050)(50) = 5.0 + 2.5 = 7.5 M−1, so [A] = 1/7.5 = 0.133 M.
⇆ Chemical Equilibrium
Q42For the reaction 4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(g), what is the relationship between Kp and Kc?
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Correct: (B)
Δn = (2 + 6) − (4 + 3) = 1, and Kp = Kc(RT)^(Δn), so Kp = Kc(RT).
Q43Consider 2NO2(g) → N2O4(g) with Kc = 4.0 at 25°C. A flask initially contains 0.80 M NO2 and no N2O4. What is the equilibrium concentration of N2O4?
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Correct: (D)
ICE: [NO2] = 0.80 − 2x, [N2O4] = x; Kc = x/(0.80 − 2x)² = 4.0. Solving gives x = 0.27 M (check: 0.27/(0.26)² = 3.99 ≈ 4.0).
Q44For PCl5(g) → PCl3(g) + Cl2(g), Kc = 0.18 at 250°C. A mixture contains [PCl5] = 0.40 M, [PCl3] = 0.30 M, and [Cl2] = 0.10 M. What will happen?
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Correct: (A)
Q = [PCl3][Cl2]/[PCl5] = (0.30)(0.10)/0.40 = 0.075 < Kc = 0.18, so the forward reaction occurs to form more products.
Q45The synthesis of ammonia, N2(g) + 3H2(g) → 2NH3(g), is exothermic (ΔH < 0). Which change will DECREASE the equilibrium yield of NH3?
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Correct: (C)
Raising the temperature shifts an exothermic equilibrium toward reactants, lowering the NH3 yield; the other changes all favor products.
Q46The solubility product for AgCl(s) → Ag+(aq) + Cl−(aq) is Ksp = 1.8×10−10 at 25°C. What is the molar solubility of AgCl in pure water?
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Correct: (B)
Ksp = s², so s = √(1.8×10−10) = 1.3×10−5 M.
Q47Which change to a system at equilibrium will change the value of the equilibrium constant K?
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Correct: (D)
Only temperature changes K; catalysts, pressure, and concentration changes alter Q (and rates) but not the value of K itself.
🧫 Acids, Bases & Buffers
Q48A 0.0050 M solution of nitric acid (HNO3) at 25°C has what pH?
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Correct: (B)
Nitric acid is a strong acid, so [H+] = 0.0050 M; pH = −log(5.0 × 10−³) = 2.30.
Q49Pyridine (C5H5N) has Kb = 1.7 × 10−⁵. What is the pH of a 0.10 M pyridine solution at 25°C?
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Correct: (D)
[OH−] = √(Kb · C) = √(1.7 × 10−¹⁰) = 1.30 × 10−⁵ M; pOH = 4.89, so pH = 14.00 − 4.89 = 9.11.
Q50Which species is the conjugate acid of HPO4²−?
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Correct: (C)
The conjugate acid has one more proton than the base: HPO4²− + H+ → H2PO4−.
Q51A buffer is prepared from 0.10 M formic acid (HCOOH, Ka = 1.8 × 10−⁴) and 0.25 M sodium formate (HCOO−). What is the pH of the buffer?
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Correct: (A)
Henderson-Hasselbalch: pH = pKa + log([base]/[acid]) = 3.74 + log(0.25/0.10) = 3.74 + 0.40 = 4.14.
Q5225.0 mL of 0.10 M acetic acid (Ka = 1.8 × 10−⁵) is titrated with 0.10 M NaOH. What is the pH at the equivalence point?
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Correct: (C)
At equivalence all acid is converted to 0.050 M CH3COO− (Kb = 5.6 × 10−¹⁰); [OH−] = √(Kb · C) = 5.3 × 10−⁶ M, pOH = 5.28, pH = 8.72 (basic, since the anion of a weak acid hydrolyzes).
Q53For oxalic acid (H2C2O4), Ka1 = 5.9 × 10−² and Ka2 = 6.4 × 10−⁵. In a 0.10 M solution at equilibrium, which species has the highest concentration (other than water)?
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Correct: (B)
Because Ka1 >> Ka2, the first proton is lost to a large extent while the second dissociation is negligible, so the intermediate species HC2O4− dominates.
🔋 Thermodynamics & Electrochemistry
Q54For which reaction is ΔS° of the system most likely to be negative?
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Correct: (A)
Four moles of gas are converted to two moles of gas, a large decrease in disorder; the other reactions increase disorder.
Q55For CO(g) + 2 H2(g) → CH3OH(g), ΔH° = −91 kJ and ΔS° = −220 J/K. At what temperature does the reaction change from spontaneous to nonspontaneous under standard conditions?
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Correct: (D)
ΔG = 0 when T = ΔH/ΔS = (−91,000 J)/( −220 J/K) = 414 K; the reaction is spontaneous below this temperature (both ΔH and ΔS negative).
Q56Magnesium metal is placed in a solution of CuSO4, and the reaction Mg(s) + Cu²⁺ → Mg²⁺ + Cu(s) occurs. Given Mg²⁺ + 2e⁻ → Mg(s), E° = −2.37 V and Cu²⁺ + 2e⁻ → Cu(s), E° = +0.34 V, what is the standard cell potential?
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Correct: (C)
E°cell = E°cathode − E°anode = (+0.34 V) − (−2.37 V) = 2.71 V.
Q57An aqueous solution of AgNO3 is electrolyzed at a constant current of 2.00 A. How many grams of silver are deposited after 40.0 minutes?
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Correct: (B)
Charge = 2.00 A × 2400 s = 4800 C; mol e− = 4800/96,485 = 0.0498; Ag+ requires 1 e− per atom, so mass = 0.0498 × 107.9 g/mol = 5.37 g.
Q58For the cell reaction Zn(s) + Cu²⁺ → Zn²⁺ + Cu(s), E°cell = 1.10 V. What is ΔG° for the reaction?
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Correct: (A)
ΔG° = −nFE° = −(2)(96,485 C/mol)(1.10 V) = −2.12 × 10⁵ J = −212 kJ.
☢ Nuclear & Descriptive Chemistry
Q59²³8U undergoes alpha decay. What is the product nucleus?
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Correct: (D)
Alpha emission removes 2 protons and 4 nucleons: A = 238 − 4 = 234, Z = 92 − 2 = 90 (thorium), giving ²³⁴Th.
Q60A 16.0 g sample of phosphorus-32 has a half-life of 14.3 days. What mass remains after 57.2 days?
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Correct: (B)
57.2/14.3 = 4 half-lives; 16.0 g → 8.0 g → 4.0 g → 2.0 g → 1.0 g.
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