ACS General Chemistry Practice Test 5
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
Q1Naturally occurring silver consists of two isotopes: Ag-107 with a mass of 106.90509 amu and an abundance of 51.839%, and Ag-109 with a mass of 108.90476 amu and an abundance of 48.161%. What is the average atomic mass of silver?
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Correct: (A)
The weighted average is (106.90509)(0.51839) + (108.90476)(0.48161) = 55.42 + 52.45 = 107.87 amu.
Q2How many unpaired electrons are present in a ground-state Fe2+ ion?
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Correct: (B)
Fe is [Ar] 4s2 3d6; Fe2+ loses the 4s electrons first, giving [Ar] 3d6, which has 4 unpaired electrons.
Q3Which of the following elements has the highest first ionization energy?
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Correct: (C)
Ionization energy generally increases across a period, and nitrogen's half-filled 2p subshell makes it harder to remove an electron than oxygen.
Q4Which of the following correctly lists the species in order of increasing radius?
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Correct: (D)
All four are isoelectronic (10 electrons); radius decreases as nuclear charge increases, so Mg2+ (12 protons) is smallest and F- (9 protons) is largest.
Q5In a sodium atom, which electron experiences the greatest effective nuclear charge?
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Correct: (A)
The 1s electrons are closest to the nucleus and experience the least shielding, so they feel nearly the full nuclear charge.
Q6An electron in a hydrogen atom falls from the n = 4 level to the n = 2 level. What is the energy of the emitted photon?
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Correct: (B)
E = 13.6 eV(1/2^2 – 1/4^2) = 13.6(0.1875) = 2.55 eV.
🔬 Chemical Bonding & Molecular Geometry
Q7What is the total number of valence electrons in the Lewis structure of the nitrate ion, NO3-?
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Correct: (C)
5 (N) + 3(6) (O) + 1 (negative charge) = 24 valence electrons.
Q8What is the molecular geometry of XeF4?
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Correct: (D)
XeF4 has 6 electron domains (4 bonding, 2 lone pairs); the lone pairs occupy opposite positions, giving square planar molecular geometry.
Q9Which of the following bonds is the most polar?
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Correct: (B)
H-F has the largest electronegativity difference (about 1.9), making it the most polar bond listed.
Q10What is the hybridization of the central atom in SF4?
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Correct: (C)
SF4 has 5 electron domains (4 bonding, 1 lone pair), requiring 5 hybrid orbitals: sp3d.
Q11What is the bond order of the superoxide ion, O2-?
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Correct: (D)
Bond order = (bonding electrons – antibonding electrons)/2 = (8 – 5)/2 = 1.5.
Q12Which of the following ionic compounds has the largest lattice energy?
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Correct: (A)
MgO contains the most highly charged ions (2+ and 2-) with small ionic radii, and lattice energy increases with charge and decreases with ion size.
Q13Which of the following molecules is nonpolar?
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Correct: (B)
BF3 is trigonal planar with symmetric bond dipoles that cancel, giving a zero net dipole moment.
🧪 Stoichiometry & Aqueous Reactions
Q1410.0 g of aluminum reacts with 25.0 g of oxygen gas according to 4 Al + 3 O2 → 2 Al2O3. What is the limiting reactant and the theoretical yield of Al2O3?
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Correct: (D)
10.0 g Al = 0.371 mol needs 0.278 mol O2, but 25.0 g O2 = 0.781 mol is present in excess; 0.371 mol Al produces 0.185 mol Al2O3 = 18.9 g.
Q15When aqueous solutions of Pb(NO3)2 and KI are mixed, what is observed?
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Correct: (A)
The double-displacement reaction produces PbI2, which is insoluble and forms a characteristic yellow precipitate.
Q16A laboratory technician dissolves 8.00 g of potassium permanganate (KMnO4, molar mass 158.03 g/mol) in water to prepare 250. mL of solution. What is the concentration of K+ ions in the resulting solution?
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Correct: (C)
8.00 g / 158.03 g/mol = 0.0506 mol KMnO4; 0.0506 mol / 0.250 L = 0.203 M, and each KMnO4 formula unit furnishes exactly one K+ ion.
Q17When the following redox reaction is balanced in acidic solution with the smallest whole-number coefficients, what is the coefficient of H+?
Cr2O7^2- + Fe^2+ → Cr^3+ + Fe^3+
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Correct: (B)
The balanced equation is Cr2O7^2- + 6 Fe^2+ + 14 H+ → 2 Cr^3+ + 6 Fe^3+ + 7 H2O; balancing O and H requires 14 H+.
Q18Ammonia is produced by the reaction N2(g) + 3 H2(g) → 2 NH3(g). If 14.0 g of N2 reacts with excess H2 and 12.5 g of NH3 is collected, what is the percent yield of NH3?
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Correct: (D)
14.0 g N2 = 0.500 mol, giving 1.00 mol NH3 theoretically = 17.0 g; (12.5 g / 17.0 g) x 100 = 73.5%.
Q19Aqueous lead(II) nitrate is mixed with aqueous potassium chromate, forming a yellow precipitate. What is the net ionic equation for this reaction?
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Correct: (A)
PbCrO4 is the insoluble precipitate; K+ and NO3- are spectator ions that cancel out.
Q20A 0.500 g sample of an unknown diprotic acid (H2A) requires 32.8 mL of 0.200 M NaOH for complete neutralization. What is the molar mass of the acid?
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Correct: (D)
0.0328 L x 0.200 mol/L = 0.00656 mol NaOH, which neutralizes 0.00328 mol of the diprotic acid; 0.500 g / 0.00328 mol = 152 g/mol.
🌀 Gases & Intermolecular Forces
Q21A 2.50 L flask contains nitrogen gas at 1.15 atm and 27°C. How many moles of N2 are present?
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Correct: (A)
n = PV/RT = (1.15 atm)(2.50 L) / (0.08206 L·atm/mol·K)(300 K) = 0.117 mol.
Q22A 5.00 L container at 25°C holds a mixture of He and O2 exerting a total pressure of 760. torr. If the partial pressure of He is 228 torr, what mass of O2 is present?
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Correct: (C)
P(O2) = 760 – 228 = 532 torr = 0.700 atm; n = (0.700)(5.00)/(0.08206)(298) = 0.143 mol; 0.143 mol x 32.00 g/mol = 4.58 g.
Q23According to kinetic molecular theory, which of the following statements is true?
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Correct: (D)
KMT postulates perfectly elastic collisions, negligible particle volume and no intermolecular forces; average kinetic energy depends only on absolute temperature.
Q24Gas X effuses 1.41 times faster than oxygen gas (O2) at the same temperature. What is the molar mass of gas X?
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Correct: (B)
By Graham's law, 1.41 = sqrt(32.00 / M); squaring gives M = 32.00 / 1.99 = 16.1 g/mol.
Q25Which pair of substances exhibits the strongest intermolecular attractions between its particles?
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Correct: (A)
Ion-dipole forces between NaCl and water are stronger than the hydrogen bonding in ammonia or the London dispersion forces in methane and carbon dioxide.
🔥 Thermochemistry
Q26For the reaction 2 SO2(g) + O2(g) → 2 SO3(g), ΔH° = -198 kJ. How much heat is released when 32.0 g of O2 reacts with excess SO2?
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Correct: (C)
32.0 g O2 = 1.00 mol O2, and the given ΔH corresponds to exactly 1 mol of O2 reacting, so 198 kJ is released.
Q27Use Hess's law to calculate ΔH for C(s) + 2 H2(g) → CH4(g), given:
C(s) + O2(g) → CO2(g) ΔH = -394 kJ
H2(g) + 1/2 O2(g) → H2O(l) ΔH = -286 kJ
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ΔH = -890 kJ
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Correct: (B)
Add the first two equations (doubling the second) and reverse the third: -394 + 2(-286) – (-890) = -76 kJ.
Q28A 25.0 g sample of an unknown metal at 95.0°C is placed in 75.0 g of water at 20.0°C in a calorimeter. The final equilibrium temperature is 24.6°C. What is the specific heat of the metal? (specific heat of water = 4.184 J/g·°C)
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Correct: (D)
Heat gained by water = (75.0)(4.184)(4.6) = 1443 J = heat lost by metal; c = 1443 J / (25.0 g x 70.4°C) = 0.820 J/g·°C.
Q29Calculate ΔH°rxn for 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g), given ΔHf° values (kJ/mol): NH3(g) = -46, NO(g) = +90, H2O(g) = -242.
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Correct: (A)
ΔH = [4(90) + 6(-242)] – [4(-46)] = (360 – 1452) + 184 = -908 kJ.
Q30How much heat is required to convert 50.0 g of ice at -10.0°C to liquid water at 40.0°C? (specific heats: ice = 2.09 J/g·°C, water = 4.184 J/g·°C; ΔHfus = 6.01 kJ/mol)
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Correct: (B)
Warming ice: 1.05 kJ; melting: (50.0/18.02 mol)(6.01 kJ/mol) = 16.7 kJ; warming water: 8.37 kJ; total = 26.1 kJ.
Q31Consider the combustion of hydrogen gas:
2H2(g) + O2(g) → 2H2O(g)
Given the bond energies: H–H = 436 kJ/mol, O=O = 498 kJ/mol, and O–H = 463 kJ/mol, what is the estimated enthalpy change (ΔH) for this reaction?
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Correct: (A)
Bonds broken: 2(436) + 498 = 1370 kJ; bonds formed: 4(463) = 1852 kJ; ΔH = 1370 − 1852 = −482 kJ.
⚡ Quantum Theory & Electronic Structure
Q32A metal surface has a work function of 2.3 eV. Light of wavelength 400 nm strikes the surface. What is the maximum kinetic energy of the ejected photoelectrons? (hc = 1240 eV·nm)
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Correct: (D)
Photon energy = 1240/400 = 3.10 eV; KE_max = 3.10 − 2.3 = 0.80 eV.
Q33An electron travels at 2.2 × 10^6 m/s. What is its de Broglie wavelength? (h = 6.626 × 10^−34 J·s; m_e = 9.11 × 10^−31 kg)
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Correct: (B)
λ = h/mv = 6.626 × 10^−34/(9.11 × 10^−31 × 2.2 × 10^6) = 3.31 × 10^−10 m.
Q34Which of the following sets of quantum numbers (n, l, m_l) describes an allowed orbital?
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Correct: (C)
Valid since l = 1 < n = 2 and |m_l| = 1 ≤ l; all other sets violate l < n or |m_l| ≤ l.
Q35A hydrogen atom electron transitions from n = 4 to n = 2. What is the wavelength of the emitted photon? (E_photon = 13.6(1/2^2 − 1/4^2) eV; hc = 1240 eV·nm)
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Correct: (A)
E = 13.6(0.1875) = 2.55 eV; λ = 1240/2.55 ≈ 486 nm.
⏱ Chemical Kinetics
Q36The following initial-rate data were collected for the reaction A + B → products:
Expt 1: [A] = 0.10 M, [B] = 0.10 M, rate = 2.0 × 10^−3 M/s
Expt 2: [A] = 0.20 M, [B] = 0.10 M, rate = 8.0 × 10^−3 M/s
Expt 3: [A] = 0.10 M, [B] = 0.20 M, rate = 8.0 × 10^−3 M/s
What is the rate law?
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Correct: (C)
Doubling [A] quadruples the rate (second order in A); doubling [B] quadruples the rate (second order in B).
Q37For a second-order reaction, k = 0.040 M^−1s^−1. If the initial concentration of reactant is 0.50 M, what is the half-life?
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Correct: (B)
t_1/2 = 1/(k[A]_0) = 1/(0.040 × 0.50) = 50 s.
Q38The rate constant of a reaction doubles when the temperature is raised from 300 K to 310 K. What is the approximate activation energy? (R = 8.314 J/mol·K)
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Correct: (D)
ln 2 = (E_a/R)(1/300 − 1/310); E_a = 0.693 × 8.314/(1.075 × 10^−4) ≈ 5.4 × 10^4 J/mol.
Q39A catalyst increases the rate of a chemical reaction primarily by
Show Answer
Correct: (B)
A catalyst lowers E_a by offering a different mechanism; it does not change ΔH or the equilibrium constant.
Q40A proposed mechanism for a reaction is:
Step 1 (slow): NO2 + NO2 → NO3 + NO
Step 2 (fast): NO3 + CO → NO2 + CO2
What is the rate law predicted by this mechanism?
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Correct: (C)
The slow (rate-determining) step involves two NO2 molecules, so rate = k[NO2]^2.
Q41The rate constant for a reaction is reported as 3.2 × 10^−3 M^−2s^−1. What is the overall order of the reaction?
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Correct: (D)
Units of k are M^(1−n)s^−1; M^−2 implies 1 − n = −2, so n = 3.
⇆ Chemical Equilibrium
Q42For the equilibrium 2SO2(g) + O2(g) ⇌ 2SO3(g), which is the correct expression for Kp?
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Correct: (B)
Kp = (partial pressures of products)^coeff/(partial pressures of reactants)^coeff = P_SO3^2/(P_SO2^2 · P_O2).
Q43For the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH° = −92 kJ/mol. Which change will increase the equilibrium yield of NH3?
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Correct: (A)
Higher pressure favors the side with fewer moles of gas (2 vs 4), shifting the equilibrium right toward NH3.
Q44For 2NO(g) + O2(g) ⇌ 2NO2(g), Kc = 4.5 × 10^13 at 298 K. What is Kp at this temperature? (R = 0.08206 L·atm/mol·K)
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Correct: (A)
Δn = 2 − 3 = −1; Kp = Kc(RT)^Δn = 4.5 × 10^13/(0.08206 × 298) ≈ 1.8 × 10^12.
Q45For CO(g) + H2O(g) ⇌ CO2(g) + H2(g), Kc = 1.5. At a particular moment, [CO] = 0.20 M, [H2O] = 0.30 M, [CO2] = 0.60 M, and [H2] = 0.30 M. Which statement is correct?
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Correct: (C)
Q = (0.60 × 0.30)/(0.20 × 0.30) = 3.0 > Kc = 1.5, so the reverse reaction is favored to reach equilibrium.
Q46For the reaction N2(g) + 3 H2(g) ⇆ 2 NH3(g), Kc = 0.45 at 500°C. A 10.0 L flask contains 0.500 mol N2, 1.50 mol H2, and 0.200 mol NH3. What happens when the system is allowed to reach equilibrium?
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Correct: (C)
Q = [NH3]^2/([N2][H2]^3) = (0.0200)^2/((0.0500)(0.150)^3) = 2.37, which is greater than Kc = 0.45, so the reaction shifts left toward reactants and [NH3] decreases.
Q47The Ksp of AgCl is 1.8 × 10^-10. What is the molar solubility of AgCl in 0.10 M NaCl(aq)?
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Correct: (D)
With [Cl-] ≈ 0.10 M from NaCl, s = Ksp/[Cl-] = (1.8 × 10^-10)/0.10 = 1.8 × 10^-9 M.
🧫 Acids, Bases & Buffers
Q48Calculate the pH of a 0.25 M solution of hypochlorous acid, HOCl (Ka = 3.5 × 10^-8).
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Correct: (B)
[H+] = √(Ka × C) = √(3.5 × 10^-8 × 0.25) = 9.4 × 10^-5 M, so pH = 4.03.
Q49Which of the following is a conjugate acid-base pair?
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Correct: (D)
H2PO4- and HPO4^2- differ by exactly one proton; the others differ by two protons or are unrelated species.
Q50A 0.15 M solution of a weak base B has a pH of 10.42. What is the Kb of the base?
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Correct: (A)
pOH = 3.58, so [OH-] = 2.6 × 10^-4 M; Kb = [OH-]^2/C = (2.6 × 10^-4)^2/0.15 = 4.6 × 10^-7.
Q51A 0.10 M solution of formic acid, HCOOH (Ka = 1.8 × 10^-4), is titrated with 0.10 M NaOH. Which statement is correct?
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Correct: (C)
At the half-equivalence point, [HA] = [A-], so pH = pKa by the Henderson-Hasselbalch equation.
Q52Which 0.10 M aqueous salt solution has the highest pH?
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Correct: (A)
CO3^2- hydrolyzes to produce OH-, making the solution basic; NH4Cl is acidic, while NaCl and KNO3 are neutral.
Q53What is the pH of a buffer containing 0.40 M NH3 and 0.20 M NH4Cl? (Kb for NH3 = 1.8 × 10^-5)
Show Answer
Correct: (D)
pKa(NH4+) = 14 – 4.74 = 9.26; pH = 9.26 + log(0.40/0.20) = 9.56.
🔋 Thermodynamics & Electrochemistry
Q54For which process is ΔS negative?
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Correct: (C)
Condensation converts a gas to a liquid, decreasing disorder, so ΔS < 0.
Q55For a reaction with ΔH° = -92.0 kJ and ΔS° = -199 J/K, at what temperature does ΔG° equal zero?
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Correct: (A)
Setting ΔG° = ΔH° – TΔS° = 0 gives T = ΔH°/ΔS° = (-92,000 J)/(-199 J/K) = 462 K.
Q56Which statement correctly distinguishes a galvanic cell from an electrolytic cell?
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Correct: (B)
This is the defining energy-conversion difference; note the anode is negative in a galvanic cell and positive in an electrolytic cell.
Q57Given: Cu^2+ + 2e- → Cu, E° = +0.34 V; Zn^2+ + 2e- → Zn, E° = -0.76 V. What is E°cell for Zn(s) + Cu^2+(aq) → Zn^2+(aq) + Cu(s)?
Show Answer
Correct: (D)
E°cell = E°cathode – E°anode = (+0.34 V) – (-0.76 V) = +1.10 V.
Q58How many grams of silver (molar mass 107.9 g/mol) are deposited when a current of 2.50 A flows for 1.00 hour through AgNO3(aq)?
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Correct: (A)
Charge = 2.50 A × 3600 s = 9000 C; moles e- = 9000/96485 = 0.0933; mass Ag = 0.0933 × 107.9 = 10.1 g.
☢ Nuclear & Descriptive Chemistry
Q59A radioactive isotope has a half-life of 12.0 days. What fraction of the original sample remains after 48.0 days?
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Correct: (B)
48.0 days is 4 half-lives, so the remaining fraction is (1/2)^4 = 1/16.
Q60Which compound is correctly matched with its observed color?
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Correct: (D)
KMnO4 solutions are deep purple; anhydrous CuSO4 is white, NaCl is white, and Fe^2+ solutions are pale green.
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