ACS General Chemistry Practice Test 2
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 chromium (Z = 24) is
Show Answer
Correct: (C)
A half-filled 4s and half-filled 3d subshell is extra stable, so one 4s electron is promoted to give [Ar] 4s^1 3d^5.
Q2Which element has the largest atomic radius?
Show Answer
Correct: (D)
Radius increases down a group; K is below Na in Group 1 and also to the left of Ca, so it is the largest.
Q3The successive ionization energies of an element are IE1 = 738, IE2 = 1451, IE3 = 7733, IE4 = 10543 kJ/mol. The element is most likely in Group
Show Answer
Correct: (A)
The large jump between IE2 and IE3 shows the third electron comes from a filled inner shell, meaning two valence electrons: Group 2.
Q4Boron has two natural isotopes: ^10B (10.013 u) and ^11B (11.009 u). If the average atomic mass of boron is 10.81 u, the percent abundance of ^10B is closest to
Show Answer
Correct: (B)
Let x = fraction of ^10B: 10.013x + 11.009(1 − x) = 10.81, so 0.996x = 0.199 and x = 0.20, or 20%.
Q5Which set of quantum numbers (n, l, ml, ms) is NOT allowed for an electron in an atom?
Show Answer
Correct: (D)
The angular momentum quantum number l must be less than n, so l = 2 is not allowed when n = 2.
Q6Which ion has the smallest ionic radius?
Show Answer
Correct: (C)
All four ions are isoelectronic; the one with the most protons, Mg^2+ (Z = 12), pulls the electrons in most tightly.
🔬 Chemical Bonding & Molecular Geometry
Q7In the best Lewis structure of SO3, the number of lone pairs on the central sulfur atom is
Show Answer
Correct: (A)
Sulfur forms three S=O double bonds (formal charges all zero), using all six valence electrons, leaving no lone pairs on S.
Q8The molecular geometry of the ammonium ion, NH4^+, is
Show Answer
Correct: (B)
Nitrogen has four bonding pairs and no lone pairs, giving a tetrahedral arrangement.
Q9Which bond is the most polar?
Show Answer
Correct: (C)
Fluorine has the largest electronegativity difference with hydrogen, so the F−H bond is the most polar.
Q10The hybridization of the carbon atom in CO2 is
Show Answer
Correct: (D)
CO2 is linear with two electron domains on carbon, requiring two hybrid orbitals: sp.
Q11Which statement about the carbonate ion, CO3^2-, is correct?
Show Answer
Correct: (B)
Resonance delocalizes the pi bond over all three C−O bonds, making them identical with bond order 1 1/3; the ion is trigonal planar with no lone pair on carbon.
Q12Which substance has the highest boiling point?
Show Answer
Correct: (A)
H2O exhibits hydrogen bonding, far stronger than the London dispersion forces in CH4, CO2, and Ne.
Q13Which molecule has the longest bond?
Show Answer
Correct: (D)
Bond length increases as bond order decreases: F2 (single bond, 142 pm) is longer than O2 (121 pm), N2 (110 pm), and H2 (74 pm).
🧪 Stoichiometry & Aqueous Reactions
Q14How many moles are in 49.0 g of H2SO4? (atomic masses: H = 1.0, S = 32.1, O = 16.0 g/mol)
Show Answer
Correct: (C)
Molar mass H2SO4 = 2(1.0) + 32.1 + 4(16.0) = 98.1 g/mol; 49.0 / 98.1 = 0.500 mol.
Q15What volume of 0.100 M NaOH is required to neutralize 25.0 mL of 0.200 M H2SO4?
Show Answer
Correct: (A)
H2SO4 + 2NaOH → Na2SO4 + 2H2O: 0.0250 L × 0.200 M = 0.00500 mol H2SO4 needs 0.0100 mol NaOH, so V = 0.0100 / 0.100 = 0.100 L = 100.0 mL.
Q16When aqueous solutions of Pb(NO3)2 and KI are mixed, the net ionic equation is
Show Answer
Correct: (B)
PbI2 precipitates while K^+ and NO3^- are spectator ions and cancel.
Q17A 50.0 mL sample of 2.00 M CuSO4 is diluted to a total volume of 250.0 mL. The concentration of the diluted solution is
Show Answer
Correct: (D)
M2 = M1V1/V2 = (2.00 × 50.0) / 250.0 = 0.400 M.
Q18A compound has the empirical formula CH2 and a molar mass of 56.0 g/mol. Its molecular formula is
Show Answer
Correct: (C)
Empirical mass CH2 = 14.0 g/mol; 56.0 / 14.0 = 4, so the molecular formula is C4H8.
Q1910.0 g of Ca (40.1 g/mol) reacts with 10.0 g of Cl2 (70.9 g/mol): Ca + Cl2 → CaCl2. Which reactant limits, and what mass of CaCl2 (110.98 g/mol) forms?
Show Answer
Correct: (A)
10.0 g Ca = 0.249 mol and 10.0 g Cl2 = 0.141 mol; the 1:1 stoichiometry makes Cl2 limiting, giving 0.141 × 110.98 = 15.6 g CaCl2.
Q20The mass percent of nitrogen in NH4NO3 is closest to (N = 14.01, H = 1.008, O = 16.00 g/mol)
Show Answer
Correct: (B)
Molar mass = 14.01 + 4(1.008) + 14.01 + 3(16.00) = 80.05 g/mol; (2 × 14.01 / 80.05) × 100% = 35.0%.
🌀 Gases & Intermolecular Forces
Q21A 3.00 L container holds 1.20 mol of O2 gas at 37°C. What is the pressure in the container? (R = 0.0821 L·atm/mol·K)
Show Answer
Correct: (C)
P = nRT/V = (1.20)(0.0821)(310)/(3.00) = 10.2 atm.
Q22A gas Y effuses at 0.500 times the rate of helium (molar mass 4.00 g/mol) under identical conditions. What is the molar mass of Y?
Show Answer
Correct: (B)
By Graham's law, (0.500)^2 = 4.00/M(Y), so M(Y) = 4.00/0.25 = 16.0 g/mol.
Q23A gas mixture of N2 and Ar has a total pressure of 4.00 atm. If the mole fraction of N2 is 0.25, what is the partial pressure of Ar?
Show Answer
Correct: (D)
P(Ar) = X(Ar) × P(total) = (1 − 0.25)(4.00 atm) = 3.00 atm.
Q24Which substance has the highest vapor pressure at 25°C?
Show Answer
Correct: (A)
Dimethyl ether cannot hydrogen bond and has the lowest boiling point, so it has the weakest intermolecular forces and the highest vapor pressure.
Q25At a given temperature, which gas has the greatest average molecular speed?
Show Answer
Correct: (C)
Average molecular speed is inversely proportional to the square root of molar mass; H2 is the lightest gas listed.
🔥 Thermochemistry
Q26A 200.0 g sample of water cools from 90.0°C to 40.0°C. What is q for the water? (specific heat of water = 4.18 J/g·°C)
Show Answer
Correct: (B)
q = mcΔT = (200.0)(4.18)(40.0 − 90.0) = −4.18 × 10^4 J = −41.8 kJ; the negative sign indicates heat lost.
Q27Given the following enthalpies:
C2H2(g) + 5/2 O2(g) → 2 CO2(g) + H2O(l) ΔH = −1299.6 kJ
C(s) + O2(g) → CO2(g) ΔH = −393.5 kJ
H2(g) + 1/2 O2(g) → H2O(l) ΔH = −285.8 kJ
What is the standard enthalpy of formation of C2H2(g)?
Show Answer
Correct: (D)
By Hess's law, ΔHf = 2(−393.5) + (−285.8) − (−1299.6) = +226.8 kJ/mol.
Q28Use average bond enthalpies to estimate ΔH for the reaction 2 H2(g) + O2(g) → 2 H2O(g). (H−H = 436 kJ/mol, O=O = 498 kJ/mol, O−H = 463 kJ/mol)
Show Answer
Correct: (A)
Bonds broken: 2(436) + 498 = 1370 kJ; bonds formed: 4(463) = 1852 kJ; ΔH = 1370 − 1852 = −482 kJ.
Q29In a coffee-cup calorimeter, 25.0 mL of 1.0 M HCl and 25.0 mL of 1.0 M NaOH, both initially at 25.0°C, are mixed. The temperature rises to 31.6°C. What is ΔH for the neutralization reaction per mole of water formed? (Assume density 1.00 g/mL and specific heat 4.18 J/g·°C for the solution.)
Show Answer
Correct: (C)
q(solution) = (50.0)(4.18)(6.6) = 1379 J; moles H2O = 0.0250; ΔH = −1.379 kJ/0.0250 mol = −55.2 kJ/mol.
Q30Which statement is true for an endothermic reaction?
Show Answer
Correct: (D)
An endothermic reaction absorbs heat from the surroundings, cooling them; ΔH is positive and the products are higher in enthalpy.
Q31Calculate ΔH for the combustion of methane: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l), given ΔHf°: CH4 = −74.8 kJ/mol, CO2 = −393.5 kJ/mol, H2O(l) = −285.8 kJ/mol.
Show Answer
Correct: (B)
ΔH = [−393.5 + 2(−285.8)] − [−74.8 + 0] = −890.3 kJ.
⚡ Quantum Theory & Electronic Structure
Q32A metal has a threshold frequency of 1.20 × 10^15 s^-1 for the photoelectric effect. Light of wavelength 180 nm strikes the metal surface. What is the kinetic energy of the ejected electron? (h = 6.626 × 10^-34 J·s, c = 3.00 × 10^8 m/s)
Show Answer
Correct: (B)
E(photon) = hc/λ = 1.10 × 10^-18 J; work function Φ = hν0 = 7.95 × 10^-19 J; KE = 1.10 × 10^-18 − 7.95 × 10^-19 = 3.09 × 10^-19 J.
Q33How many orbitals are in the n = 4 principal shell, and what is the maximum number of electrons it can hold?
Show Answer
Correct: (D)
Number of orbitals = n^2 = 16; maximum electrons = 2n^2 = 32.
Q34What is the de Broglie wavelength of a proton moving at 2.00 × 10^5 m/s? (h = 6.626 × 10^-34 J·s, mass of proton = 1.67 × 10^-27 kg)
Show Answer
Correct: (C)
λ = h/mv = (6.626 × 10^-34)/(1.67 × 10^-27)(2.00 × 10^5) = 1.98 × 10^-12 m.
Q35An electron in a hydrogen atom falls from the n = 4 level to the n = 2 level. What is the wavelength of the emitted photon? (Rydberg constant R = 1.097 × 10^7 m^-1)
Show Answer
Correct: (A)
1/λ = R(1/2^2 − 1/4^2) = (1.097 × 10^7)(3/16) = 2.06 × 10^6 m^-1, so λ = 4.86 × 10^-7 m = 486 nm.
⏱ Chemical Kinetics
Q36For the reaction 2 X + Y → products, the following initial-rate data were collected.
Experiment 1: [X] = 0.20 M, [Y] = 0.10 M, initial rate = 4.0 × 10^-4 M/s
Experiment 2: [X] = 0.40 M, [Y] = 0.10 M, initial rate = 4.0 × 10^-4 M/s
Experiment 3: [X] = 0.20 M, [Y] = 0.20 M, initial rate = 1.6 × 10^-3 M/s
What is the rate law for this reaction?
Show Answer
Correct: (D)
Doubling [X] does not change the rate (zero order in X); doubling [Y] quadruples the rate (second order in Y).
Q37A first-order reaction has a rate constant of 1.4 × 10^-2 s^-1. What is the half-life of the reactant?
Show Answer
Correct: (A)
t1/2 = 0.693/k = 0.693/(1.4 × 10^-2) = 49.5 s ≈ 50 s.
Q38A proposed mechanism for 2 NO(g) + Br2(g) → 2 NOBr(g) is:
Step 1: NO + Br2 ⇌ NOBr2 (fast equilibrium)
Step 2: NOBr2 + NO → 2 NOBr (slow)
What rate law is predicted by this mechanism?
Show Answer
Correct: (C)
The slow step gives Rate = k[NOBr2][NO]; from the fast pre-equilibrium, [NOBr2] = K[NO][Br2], so Rate = k'[NO]^2[Br2].
Q39A first-order reaction has k = 0.050 s^-1. If the initial concentration of reactant is 1.20 M, what is the concentration after 30.0 s?
Show Answer
Correct: (B)
ln[A] = ln(1.20) − (0.050)(30) = 0.182 − 1.50 = −1.32, so [A] = e^−1.32 = 0.27 M.
Q40A reaction has the rate law Rate = k[X]^2[Y]. What are the units of the rate constant k?
Show Answer
Correct: (D)
The reaction is third order overall, so k must have units of M^(1−3)·s^-1 = M^-2·s^-1 for the rate to come out in M/s.
Q41For most reactions, raising the temperature by 10°C approximately doubles the reaction rate. This observation is best explained by the fact that at higher temperature:
Show Answer
Correct: (A)
The rate increase comes mainly from the exponential increase in the fraction of molecules with enough energy to react, not from a modest increase in collision frequency.
⇆ Chemical Equilibrium
Q42For the reaction CaCO3(s) ⇌ CaO(s) + CO2(g), what is the correct expression for Kc?
Show Answer
Correct: (B)
Pure solids are omitted from equilibrium expressions, leaving only the gaseous product.
Q43For 2 NO(g) + Br2(g) ⇌ 2 NOBr(g), the equilibrium concentrations are [NO] = 0.10 M, [Br2] = 0.20 M, and [NOBr] = 0.60 M. What is the value of Kc?
Show Answer
Correct: (D)
Kc = [NOBr]^2/([NO]^2[Br2]) = (0.60)^2/((0.10)^2(0.20)) = 0.36/0.002 = 180.
Q44For 2 H2(g) + O2(g) ⇌ 2 H2O(g), Kc = 2.4 × 10^47 at 500 K. What is Kp at this temperature? (R = 0.0821 L·atm/mol·K)
Show Answer
Correct: (C)
Δn = 2 − 3 = −1, so Kp = Kc(RT)^−1 = (2.4 × 10^47)/(0.0821 × 500) = 5.8 × 10^45.
Q45Consider N2(g) + O2(g) ⇌ 2 NO(g), ΔH° = +181 kJ/mol. Which change will increase the equilibrium concentration of NO?
Show Answer
Correct: (A)
The reaction is endothermic, so higher temperature shifts equilibrium toward products; pressure has no effect since Δn = 0.
Q46For PCl5(g) ⇌ PCl3(g) + Cl2(g), Kc = 0.045 at 250°C. A flask initially contains 0.60 M PCl5 and no products. What is [Cl2] at equilibrium?
Show Answer
Correct: (B)
x^2/(0.60 − x) = 0.045 gives x^2 + 0.045x − 0.027 = 0, so x = 0.14 M = [Cl2].
Q47For CO(g) + H2O(g) ⇌ CO2(g) + H2(g), Kc = 5.1 at 700 K. A mixture initially contains [CO] = 0.20 M, [H2O] = 0.20 M, [CO2] = 0.10 M, and [H2] = 0.10 M. Which statement is correct?
Show Answer
Correct: (C)
Q = ([CO2][H2])/([CO][H2O]) = (0.10)(0.10)/((0.20)(0.20)) = 0.25; since Q < K, the reaction proceeds toward products.
🧫 Acids, Bases & Buffers
Q48What is the pOH of a 0.0020 M Ba(OH)2 solution? (Assume complete dissociation.)
Show Answer
Correct: (C)
[OH^-] = 2(0.0020) = 0.0040 M; pOH = −log(4.0×10^-3) = 2.40.
Q49Which of the following aqueous mixtures acts as a buffer?
Show Answer
Correct: (A)
It contains the weak acid HCN and its conjugate base CN^- in comparable amounts.
Q50A 0.050 M solution of the weak base NH3 has Kb = 1.8×10^-5. What is the pH of the solution?
Show Answer
Correct: (D)
[OH^-] = sqrt(Kb·C) = 9.5×10^-4 M, so pOH = 3.02 and pH = 14.00 − 3.02 = 10.98.
Q51For nitrous acid, HNO2, Ka = 4.5×10^-4. What is Kb for its conjugate base, NO2^-? (Kw = 1.0×10^-14)
Show Answer
Correct: (B)
Kb = Kw/Ka = (1.0×10^-14)/(4.5×10^-4) = 2.2×10^-11.
Q5220.0 mL of 0.10 M HCl is titrated with 0.10 M NaOH. What is the pH at the equivalence point?
Show Answer
Correct: (D)
A strong acid titrated with a strong base gives a neutral salt solution at equivalence.
Q53A buffer is prepared with [NH3] = 0.020 M and [NH4^+] = 0.20 M. If pKa = 9.25 for NH4^+, what is the pH?
Show Answer
Correct: (C)
pH = 9.25 + log(0.020/0.20) = 9.25 − 1.00 = 8.25.
🔋 Thermodynamics & Electrochemistry
Q54What is the sign of ΔS° for the reaction CaCO3(s) → CaO(s) + CO2(g)?
Show Answer
Correct: (B)
A gas is produced from a solid with no gaseous reactant, so disorder increases.
Q55A reaction has ΔH° = +85 kJ/mol and ΔS° = +0.250 kJ/(mol·K). At which temperatures is the reaction spontaneous?
Show Answer
Correct: (A)
ΔG = 0 at T = 85/0.250 = 340 K; with ΔH > 0 and ΔS > 0, ΔG < 0 only above 340 K.
Q56For the reaction Ni(s) + 2 Ag^+(aq) → Ni^2+(aq) + 2 Ag(s), E° = +0.80 V for Ag^+/Ag and E° = −0.25 V for Ni^2+/Ni. What is E°cell?
Show Answer
Correct: (D)
E°cell = E°cathode − E°anode = 0.80 − (−0.25) = 1.05 V.
Q57In the galvanic cell Zn|Zn^2+||Cu^2+|Cu, the concentration of Zn^2+ is increased while [Cu^2+] is held constant. What happens to the cell potential?
Show Answer
Correct: (C)
By the Nernst equation, E = E° − (RT/nF) ln([Zn^2+]/[Cu^2+]); increasing [Zn^2+] raises Q and lowers E.
Q58What quantity of electric charge is required to deposit 9.0 g of aluminum from an Al^3+ solution? (Al^3+ + 3e^- → Al; molar mass of Al = 26.98 g/mol; F = 96,485 C/mol e^-)
Show Answer
Correct: (B)
Moles Al = 9.0/26.98 = 0.334 mol, requiring 3(0.334) = 1.00 mol e^-, so Q = 1.00 × 96,485 = 9.65×10^4 C.
☢ Nuclear & Descriptive Chemistry
Q59Which particle is emitted from the nucleus during positron emission?
Show Answer
Correct: (A)
Positron emission ejects a positron (^0_+1e) as a proton converts into a neutron.
Q60A 120 g sample of a radioisotope with a half-life of 8.0 hours decays for 24 hours. What mass remains?
Show Answer
Correct: (C)
24/8.0 = 3 half-lives, so 120 × (1/2)^3 = 15.0 g remains.
Want more practice?