ACS General Chemistry Practice Test 10
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
Q1Antimony has two naturally occurring isotopes: Sb-121 (mass 120.904 u, 57.2% abundance) and Sb-123 (mass 122.904 u). What is the average atomic mass of antimony?
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Correct: (B)
0.572(120.904) + 0.428(122.904) = 69.16 + 52.60 = 121.76 u.
Q2Which of the following is the correct ground-state electron configuration for the Cu+ ion?
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Correct: (C)
Neutral Cu is [Ar] 4s1 3d10; the ion loses the 4s electron first, giving [Ar] 3d10.
Q3Which species is isoelectronic with the sulfide ion, S2−?
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Correct: (C)
S2− has 18 electrons; Cl− (18 e−), Ar (18 e−), and K+ (18 e−) are all isoelectronic with it.
Q4Which of the following correctly orders the atoms by increasing atomic radius?
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Correct: (A)
Atomic radius decreases across a period as effective nuclear charge increases, so C > N > O > F; increasing order is F < O < N < C.
Q5Which element has the highest electronegativity?
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Correct: (D)
Electronegativity increases across a period and decreases down a group. Among these choices, Br (2.96) has the highest Pauling value, followed by S (2.58), P (2.19), and Si (1.90).
Q6An element exhibits the following successive ionization energies (kJ/mol): IE1 = 578, IE2 = 1817, IE3 = 2745, IE4 = 11577. To which group does this element most likely belong?
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Correct: (C)
The large jump occurs between IE3 and IE4, meaning three valence electrons are removed before a core electron is reached — characteristic of Group 3A.
🔬 Chemical Bonding & Molecular Geometry
Q7In the best Lewis structure of the nitrite ion, NO2− (obeying the octet rule with maximum nonbonding electrons on the terminal atoms), how many resonance structures can be drawn?
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Correct: (B)
NO2− has 18 valence electrons; the N=O double bond can be placed on either oxygen, giving 2 equivalent resonance structures.
Q8What is the formal charge on each oxygen atom in the Lewis structure of ozone, O3, drawn with one double bond and one single bond?
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Correct: (B)
In O3 the central O has formal charge +1; the double-bonded terminal O has 6 − (4 + 2) = 0, and the single-bonded terminal O has 6 − (6 + 1) = −1.
Q9According to VSEPR theory, what is the molecular geometry of xenon tetrafluoride, XeF4?
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Correct: (B)
XeF4 has 4 bonding pairs and 2 lone pairs (AX4E2); the lone pairs occupy axial positions, giving square planar molecular geometry.
Q10Which of the following molecules has the smallest bond angle?
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Correct: (C)
BeCl2 is linear (180°), BCl3 is trigonal planar (120°), CCl4 is tetrahedral (109.5°), and H2S is bent with two lone pairs compressing the angle to about 92°.
Q11What is the hybridization of the carbon atoms in a molecule of ethyne (acetylene), C2H2?
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Correct: (C)
Each carbon in HC≡CH forms 2 σ bonds and 2 π bonds (linear geometry, 180°), requiring sp hybridization.
Q12Which of the following bonds is the most polar?
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Correct: (D)
Bond polarity increases with the electronegativity difference; fluorine is the most electronegative element, so F−H has the largest ΔEN.
Q13Which of the following correctly ranks the carbon–oxygen bonds in order of increasing bond strength: CO (carbon monoxide), CO2, CH3OH (methanol)?
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Correct: (A)
Bond orders: C−O single in CH3OH (1), C=O double in CO2 (2), C≡O triple in CO (3). Bond strength increases with bond order: CH3OH < CO2 < CO.
🧪 Stoichiometry & Aqueous Reactions
Q14How many oxygen atoms are present in 15.0 g of potassium sulfate, K2SO4? (Molar mass K2SO4 = 174.26 g/mol)
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Correct: (C)
15.0 g / 174.26 g/mol = 0.0861 mol K2SO4 = 5.18×10²² formula units; each unit has 4 O atoms, so 4 × 5.18×10²² = 2.07×10²³ O atoms.
Q15Copper is displaced from solution by aluminum: 2Al(s) + 3CuSO4(aq) → Al2(SO4)3(aq) + 3Cu(s). If 10.8 g of Al reacts with 63.8 g of CuSO4, which reactant limits and what mass of Cu forms? (Molar masses: Al = 26.98, CuSO4 = 159.62, Cu = 63.55 g/mol)
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Correct: (B)
10.8 g Al = 0.400 mol requires 0.600 mol CuSO4, but only 0.400 mol CuSO4 is present, so CuSO4 limits; 0.400 mol Cu × 63.55 g/mol = 25.4 g Cu.
Q16Titanium is produced by TiCl4(l) + 2Mg(s) → Ti(s) + 2MgCl2(s). If 95.0 g of TiCl4 produces 19.2 g of Ti, what is the percent yield? (Molar masses: TiCl4 = 189.67, Ti = 47.87 g/mol)
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Correct: (D)
Theoretical yield: 95.0 g TiCl4 = 0.501 mol → 0.501 mol Ti = 24.0 g; percent yield = (19.2/24.0) × 100% = 80.1%.
Q17A compound contains 52.2% C, 13.0% H, and 34.8% O by mass, with a molar mass of 46.07 g/mol. What is its molecular formula?
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Correct: (A)
The mass percentages give a C:H:O mole ratio of 2:6:1 (empirical formula C2H6O, empirical mass 46.07 g/mol), which equals the molar mass, so the molecular formula is C2H6O.
Q18What volume of 12.0 M NaOH stock solution is required to prepare 500. mL of 0.750 M NaOH?
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Correct: (B)
M1V1 = M2V2 gives V1 = (0.500 L)(0.750 M)/(12.0 M) = 0.03125 L = 31.3 mL.
Q19What volume of 0.150 M AgNO3 is required to completely precipitate all the chloride from 25.0 mL of 0.200 M MgCl2? (2AgNO3 + MgCl2 → 2AgCl(s) + Mg(NO3)2)
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Correct: (D)
Moles Cl^- = 2(0.0250 L)(0.200 M) = 0.0100 mol, requiring 0.0100 mol AgNO3; V = 0.0100 mol / 0.150 M = 0.0667 L = 66.7 mL.
Q20Balance the following redox reaction in acidic solution: Cr2O7^2- + Fe^2+ → Cr^3+ + Fe^3+. What is the coefficient of H^+ in the balanced equation?
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Correct: (C)
The balanced equation is Cr2O7^2- + 6Fe^2+ + 14H^+ → 2Cr^3+ + 6Fe^3+ + 7H2O; the dichromate reduction requires 14 H^+ to balance the 7 oxygen atoms as water.
🌀 Gases & Intermolecular Forces
Q21Under which set of conditions does a real gas deviate most from ideal behavior?
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Correct: (A)
High pressure forces molecules close together (finite molecular volume matters) and low temperature reduces kinetic energy so attractive forces matter; both increase deviation from ideality.
Q22In which substance is hydrogen bonding the strongest intermolecular force?
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Correct: (B)
Methylamine contains N–H bonds, so its molecules form hydrogen bonds; CH4, CO2, and BF3 are nonpolar and held together only by London dispersion forces.
Q23The triple point of CO2 is −56.6°C and 5.11 atm. Which statement is correct?
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Correct: (D)
Since 1.00 atm is below the triple-point pressure of 5.11 atm, the liquid phase cannot exist at 1.00 atm, so solid CO2 (dry ice) sublimes.
Q24Samples of O2 and H2 are each at 300 K. Which statement is true?
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Correct: (A)
Kinetic molecular theory states that average kinetic energy depends only on temperature (KEavg = 3/2 RT), so it is identical for both gases at 300 K; the lighter H2 molecules move faster, but their average KE is the same.
Q25Which compound has the highest boiling point?
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Correct: (C)
1-Butanol can form hydrogen bonds (–OH group), the strongest intermolecular force among these choices; the others rely on weaker dipole-dipole or London dispersion forces, so 1-butanol boils highest.
🔥 Thermochemistry
Q26When 0.0100 mol of an ionic salt dissolves in 100.0 g of water in a coffee-cup calorimeter, the temperature rises from 24.0 °C to 31.2 °C. Estimate the molar enthalpy of solution (ΔHsoln) of the salt.
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Correct: (C)
q absorbed by water = 100.0(4.184)(31.2 − 24.0) = 3012 J = 3.01 kJ. The reaction released this heat, so ΔHsoln = −3.01 kJ/0.0100 mol = −301 kJ/mol (exothermic).
Q27Use Hess's law with the reactions below to find the standard enthalpy of formation of liquid ethanol, 2C(s) + 3H2(g) + ½O2(g) → C2H5OH(l).
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) ΔH = −1366.8 kJ
C(s) + O2(g) → CO2(g) ΔH = −393.5 kJ
H2(g) + ½O2(g) → H2O(l) ΔH = −285.8 kJ
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Correct: (D)
Reverse the combustion (×1): +1366.8 kJ; double the second: −787.0 kJ; triple the third: −857.4 kJ. CO2, H2O, and excess O2 cancel, giving the formation reaction with ΔH = 1366.8 − 787.0 − 857.4 = −277.6 kJ.
Q28For the equilibrium N2O4(g) → 2NO2(g), use ΔHf° [NO2(g)] = 33.2 kJ/mol and ΔHf° [N2O4(g)] = 9.16 kJ/mol. What is ΔH°rxn?
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Correct: (B)
ΔH° = 2(33.2) − 9.16 = 66.4 − 9.16 = 57.2 kJ. The positive sign correctly indicates the dissociation is endothermic.
Q29How much total heat is required to convert 40.0 g of liquid water at 20.0 °C completely to steam at 100.0 °C? (c(water) = 4.184 J/g·°C; ΔHvap = 40.7 kJ/mol)
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Correct: (A)
Heating: 40.0(4.184)(80.0) = 13,389 J = 13.4 kJ. Vaporization: (40.0/18.015)(40.7) = 2.220(40.7) = 90.4 kJ. Total = 13.4 + 90.4 = 103.8 ≈ 104 kJ.
Q30Estimate ΔH for 2H2(g) + O2(g) → 2H2O(g) using average bond enthalpies: H–H 436, O=O 498, O–H 463 kJ/mol.
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Correct: (C)
Broken: 2 H–H + 1 O=O = 872 + 498 = 1370 kJ. Formed: 4 O–H = 1852 kJ. ΔH = 1370 − 1852 = −482 kJ (for 2 mol H2O formed).
Q31An instant cold pack becomes cold when ammonium nitrate dissolves in water. Which statement is true about this process?
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Correct: (B)
The pack cools its surroundings, meaning heat flows from the surroundings into the system; the system gains enthalpy, so ΔH > 0 (endothermic). The cold sensation is the surroundings losing heat.
⚡ Quantum Theory & Electronic Structure
Q32Red light has a frequency (ν) of 4.60 x 10^14 Hz. What is its wavelength? (c = 2.998 x 10^8 m/s)
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Correct: (A)
λ = c/ν = 2.998 x 10^8 / 4.60 x 10^14 = 6.52 x 10^-7 m = 652 nm.
Q33In the hydrogen emission spectrum, an electron falls from the n = 4 level to the n = 2 level (Balmer series). What is the wavelength of the emitted photon? (Rydberg constant R = 1.097 x 10^7 m^-1)
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Correct: (B)
1/λ = R(1/2² − 1/4²) = 1.097 x 10^7 (1/4 − 1/16) = 2.057 x 10^6 m^-1, so λ = 4.86 x 10^-7 m = 486 nm.
Q34Which statement best expresses the Heisenberg uncertainty principle?
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Correct: (A)
The principle states that Δx · Δp ≥ h/4π: sharpening knowledge of position inherently blurs knowledge of momentum, and vice versa. It is a fundamental limit, not an instrument flaw.
Q35An electron is described as occupying a 4d orbital. Which quantum numbers (n, l) does it have?
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Correct: (D)
The principal quantum number n = 4 comes from the "4", and a d orbital corresponds to l = 2 (s = 0, p = 1, d = 2, f = 3).
⏱ Chemical Kinetics
Q36Consider the reaction A(g) + 2B(g) → C(g). Three initial-rate experiments gave:
Exp 1: [A] = 0.20 M, [B] = 0.20 M, rate = 1.2×10−3 M/s
Exp 2: [A] = 0.40 M, [B] = 0.20 M, rate = 2.4×10−3 M/s
Exp 3: [A] = 0.20 M, [B] = 0.40 M, rate = 4.8×10−3 M/s
What is the rate law?
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Correct: (A)
Doubling [A] (Exp 1 → 2) doubles the rate (1st order in A); doubling [B] (Exp 1 → 3) quadruples the rate (2nd order in B).
Q37The first-order decomposition of cyclopropane has k = 3.0×10−4 s−1 at 750 K. Starting with [cyclopropane] = 0.60 M, what is the concentration after 1.00 h?
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Correct: (C)
ln([A]0/[A]) = kt = (3.0×10−4)(3600) = 1.08, so [A] = 0.60/e^1.08 = 0.60/2.94 = 0.20 M.
Q38The rate constant for the isomerization of methyl isocyanide is 5.0×10−4 s−1 at 300 K. If the activation energy is 65.0 kJ/mol, what is the rate constant at 320 K?
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Correct: (D)
ln(k2/k1) = (Ea/R)(1/T1 − 1/T2) = (65000/8.314)(1/300 − 1/320) = 1.63, so k2 = 5.0×10−4 × e^1.63 = 2.6×10−3 s−1.
Q39Ozone destruction in the stratosphere is proposed to occur by this mechanism:
Step 1: NO(g) + O3(g) → NO2(g) + O2(g) (slow)
Step 2: NO2(g) + O(g) → NO(g) + O2(g) (fast)
What is the rate law predicted by this mechanism?
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Correct: (A)
The slow (rate-determining) step involves one NO and one O3 colliding, so its molecularity gives rate = k[NO][O3].
Q40For a reaction that is second order in reactant A, which plot yields a straight line?
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Correct: (C)
The integrated second-order law 1/[A]t = kt + 1/[A]0 is linear in 1/[A] vs. time; ln[A] vs. time is linear for first order.
Q41Increasing the temperature of a reaction mixture increases the reaction rate primarily because it:
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Correct: (B)
The dominant effect of higher T is the exponential increase in the fraction of molecules that can surmount the activation barrier.
⇆ Chemical Equilibrium
Q42For the reaction H2(g) + Br2(g) → 2HBr(g), the correct expression for Kc is:
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Correct: (C)
Kc = [products]^coefficients/[reactants]^coefficients = [HBr]²/([H2][Br2]).
Q43For CO(g) + Cl2(g) → COCl2(g), what is the relationship between Kp and Kc?
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Correct: (D)
Δn = 1 − 2 = −1, so Kp = Kc(RT)^(−1) = Kc/(RT).
Q44Consider CaCO3(s) → CaO(s) + CO2(g), ΔH > 0. Which change will DECREASE the equilibrium pressure of CO2?
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Correct: (B)
The decomposition is endothermic, so lowering the temperature shifts the equilibrium left toward CaCO3, reducing the CO2 pressure.
Q45For H2(g) + I2(g) → 2HI(g), Kc = 54 at 425°C. A mixture contains [H2] = 0.10 M, [I2] = 0.10 M, and [HI] = 1.0 M. What will occur?
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Correct: (B)
Q = [HI]²/([H2][I2]) = (1.0)²/(0.10 × 0.10) = 100 > Kc = 54, so the reverse reaction occurs to form more reactants.
Q46Ksp for Mg(OH)2 is 5.6×10−12 at 25°C. What is the molar solubility of Mg(OH)2 in 0.10 M NaOH?
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Correct: (D)
Ksp = [Mg²+][OH−]² = s(0.10)² (common-ion from NaOH dominates [OH−]), so s = 5.6×10−12/0.010 = 5.6×10−10 M.
Q47For the equilibrium 2CO(g) + O2(g) → 2CO2(g), which change shifts the equilibrium toward products?
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Correct: (A)
There are 3 moles of gas on the reactant side but only 2 on the product side, so increasing pressure (decreasing volume) favors products.
🧫 Acids, Bases & Buffers
Q48A 0.015 M solution of Ba(OH)2 at 25°C has what pH?
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Correct: (C)
Ba(OH)2 furnishes 2 OH− per formula unit, so [OH−] = 0.030 M; pOH = −log(0.030) = 1.52, pH = 14.00 − 1.52 = 12.48.
Q49Benzoic acid (C6H5COOH) has Ka = 6.3 × 10−⁵. What is the pH of a 0.20 M solution at 25°C?
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Correct: (A)
[H+] = √(Ka · C) = √(6.3 × 10−⁵ × 0.20) = 3.55 × 10−³ M; pH = 2.45.
Q50Which of the following pairs is NOT a conjugate acid-base pair?
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Correct: (D)
The conjugate base of H2SO4 is HSO4− (differs by one H+), not SO3²−, which differs by H2O; the other three differ by exactly one proton.
Q51A buffer contains 0.30 M NH3 (Kb = 1.8 × 10−⁵) and 0.20 M NH4Cl. What is the pH of the buffer?
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Correct: (B)
pKa(NH4+) = 14.00 − 4.74 = 9.26; pH = 9.26 + log(0.30/0.20) = 9.26 + 0.18 = 9.43.
Q5220.0 mL of 0.15 M NH3 is titrated with 0.15 M HCl. What is the pH at the equivalence point?
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Correct: (A)
At equivalence the solution is 0.075 M NH4+ (Ka = 5.6 × 10−¹⁰); [H+] = √(Ka · C) = 6.5 × 10−⁶ M, pH = 5.19 (acidic, since the cation of a weak base hydrolyzes).
Q53For the diprotic acid H2X, Ka1 = 1.0 × 10−³ and Ka2 = 1.0 × 10−⁸. Which species predominates at pH 6.5?
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Correct: (B)
pKa1 = 3.0 and pKa2 = 7.0; pH 6.5 lies between the two pKa values, so the intermediate species HX− is the major form.
🔋 Thermodynamics & Electrochemistry
Q54Calculate ΔS° for the reaction 2 H2O(l) → 2 H2(g) + O2(g). Given S° (J/mol·K): H2O(l) = 69.9, H2(g) = 130.6, O2(g) = 205.0.
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Correct: (C)
ΔS° = (2 × 130.6 + 205.0) − (2 × 69.9) = 466.2 − 139.8 = +326.4 J/K.
Q55For H2O(l) → H2O(g), ΔH° = +44 kJ and ΔS° = +119 J/K. Above what temperature is the process spontaneous under standard conditions?
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Correct: (B)
ΔG = 0 at T = ΔH/ΔS = 44,000/119 = 370 K; with both ΔH and ΔS positive, the process is spontaneous above this temperature.
Q56A galvanic cell uses the half-reactions Ag+ + e⁻ → Ag(s), E° = +0.80 V and Cu²⁺ + 2e⁻ → Cu(s), E° = +0.34 V. What is E°cell?
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Correct: (A)
Silver is reduced (cathode) and copper is oxidized (anode): E°cell = 0.80 V − 0.34 V = 0.46 V.
Q57Water is electrolyzed with a current of 2.00 A for 3.00 hours. What volume of O2 gas is collected at STP?
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Correct: (D)
Charge = 2.00 A × 10,800 s = 21,600 C; mol e− = 0.2239; O2 requires 4 e− per molecule, so mol O2 = 0.0560; V = 0.0560 × 22.4 L/mol = 1.25 L.
Q58Which statement correctly describes an operating galvanic cell?
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Correct: (C)
In a galvanic cell oxidation occurs at the (negative) anode and reduction at the (positive) cathode, with electrons flowing anode → cathode externally; choice B describes an electrolytic cell and D reverses the anion direction.
☢ Nuclear & Descriptive Chemistry
Q59In the first artificial nuclear transmutation, nitrogen-14 was bombarded with alpha particles: ¹⁴N + ⁴He → ¹⁷O + ? What is the missing particle?
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Correct: (A)
Mass numbers: 14 + 4 = 17 + 1; atomic numbers: 7 + 2 = 8 + 1, so the missing particle has A = 1 and Z = 1, a proton (¹p).
Q60Which oxide forms a basic solution when dissolved in water?
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Correct: (B)
Na2O + H2O → 2 NaOH, a strong base; CO2, SO3, and P4O10 are acidic (nonmetal) oxides that form acids in water.
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