ACS General Chemistry Practice Test 6
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
Q1An atom of the isotope Se-79 contains how many neutrons?
Show Answer
Correct: (B)
Selenium has atomic number 34, so neutrons = 79 – 34 = 45.
Q2Which element has the ground-state electron configuration [Ar] 4s1 3d5?
Show Answer
Correct: (D)
Chromium is the well-known exception to the expected [Ar] 4s2 3d4 configuration, adopting [Ar] 4s1 3d5 for a half-filled d subshell.
Q3Which of the following elements has the largest electron affinity?
Show Answer
Correct: (A)
Although electron affinity generally increases up a group, chlorine exceeds fluorine because fluorine's small size causes greater electron-electron repulsion when the extra electron is added.
Q4Which of the following correctly lists the elements in order of increasing atomic radius?
Show Answer
Correct: (C)
Atomic radius decreases across a period as effective nuclear charge increases: Cl < S < P < Si.
Q5In a phosphorus atom, which electron experiences the smallest effective nuclear charge?
Show Answer
Correct: (D)
The 3p valence electrons are farthest from the nucleus and most shielded by inner electrons, so they feel the smallest effective nuclear charge.
Q6Which electronic transition in a hydrogen atom emits light of the shortest wavelength?
Show Answer
Correct: (D)
The shortest wavelength corresponds to the largest energy drop, which is the n = 5 to n = 1 transition (Lyman series).
🔬 Chemical Bonding & Molecular Geometry
Q7In the best Lewis structure of carbon monoxide, CO, what are the formal charges on carbon and oxygen?
Show Answer
Correct: (A)
In the :C≡O:+ structure, carbon has one lone pair and three bonds (formal charge -1) and oxygen has one lone pair and three bonds (formal charge +1).
Q8Which of the following molecules has bond angles closest to 120°?
Show Answer
Correct: (C)
SO3 is trigonal planar with bond angles of 120°.
Q9How many lone pairs are on the xenon atom in the Lewis structure of XeF2?
Show Answer
Correct: (B)
XeF2 has a linear geometry with three lone pairs on the central xenon atom (AX2E3); each fluorine also has three lone pairs, but the question asks only about xenon.
Q10Using bond energies H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, and H-Cl = 431 kJ/mol, estimate ΔH° for the reaction H2(g) + Cl2(g) → 2 HCl(g).
Show Answer
Correct: (B)
ΔH = (436 + 242) – 2(431) = 678 – 862 = -184 kJ.
Q11Which of the following species exhibits resonance?
Show Answer
Correct: (A)
The carbonate ion has three equivalent C-O bonds that cannot be represented by a single Lewis structure, requiring three resonance forms.
Q12Rank the following ionic compounds in order of increasing lattice energy.
Show Answer
Correct: (C)
Lattice energy increases with cation charge for a common anion: Na+ (1+) < Mg2+ (2+) < Al3+ (3+).
Q13Which of the following molecules contains the shortest bond?
Show Answer
Correct: (D)
H2 has the smallest atoms and a short single bond (~74 pm), shorter than any of the halogen-halogen bonds.
🧪 Stoichiometry & Aqueous Reactions
Q14What volume of 6.0 M HCl is required to prepare 500 mL of 0.50 M HCl?
Show Answer
Correct: (A)
Using M1V1 = M2V2: V1 = (0.50 M)(0.500 L)/(6.0 M) = 0.0417 L = 41.7 mL.
Q15What mass of AgCl precipitates when 50.0 mL of 0.200 M AgNO3 reacts with excess NaCl solution?
Show Answer
Correct: (C)
Moles Ag+ = (0.0500 L)(0.200 mol/L) = 0.0100 mol; mass AgCl = (0.0100 mol)(143.3 g/mol) = 1.43 g.
Q16A stock solution of 6.00 M HCl is used to prepare 750. mL of 0.250 M HCl. What volume of the stock solution is required?
Show Answer
Correct: (D)
M1V1 = M2V2 gives V1 = (0.250 M)(750. mL) / 6.00 M = 31.3 mL.
Q17Solutions of ammonium sulfide and copper(II) sulfate are mixed, forming a precipitate. What is the net ionic equation for this reaction?
Show Answer
Correct: (B)
CuS is the insoluble precipitate; NH4+ and SO4^2- remain in solution as spectator ions.
Q18When the reaction MnO4- + SO3^2- → MnO2 + SO4^2- is balanced in basic solution with the smallest whole-number coefficients, what is the coefficient of H2O?
Show Answer
Correct: (A)
The balanced equation in base is 2 MnO4- + 3 SO3^2- + H2O → 2 MnO2 + 3 SO4^2- + 2 OH-; H2O has a coefficient of 1.
Q19A 25.00 mL sample of vinegar is titrated with 0.500 M NaOH; 18.75 mL is required to reach the phenolphthalein endpoint. What is the molarity of acetic acid (HC2H3O2) in the vinegar?
Show Answer
Correct: (C)
0.01875 L x 0.500 mol/L = 0.00938 mol NaOH = 0.00938 mol acetic acid (1:1); 0.00938 mol / 0.02500 L = 0.375 M.
Q20Calcium carbonate decomposes according to CaCO3(s) → CaO(s) + CO2(g). Heating 50.0 g of CaCO3 yields 19.5 g of CaO. What is the percent yield of CaO?
Show Answer
Correct: (D)
50.0 g CaCO3 = 0.500 mol, giving 0.500 mol CaO = 28.0 g theoretically; (19.5 g / 28.0 g) x 100 = 69.6%.
🌀 Gases & Intermolecular Forces
Q21Which of the following gases has the greatest density at STP?
Show Answer
Correct: (B)
At the same T and P, gas density is proportional to molar mass; SO2 (64.07 g/mol) is the heaviest of the four.
Q22A sample of an unknown gas effuses through a pinhole in 48.0 s, while the same volume of neon effuses in 30.0 s under identical conditions. What is the approximate molar mass of the unknown gas?
Show Answer
Correct: (A)
Effusion time is proportional to sqrt(M), so M = 20.18 g/mol x (48.0/30.0)^2 = 51.7 g/mol.
Q23At a given temperature, which liquid would have the highest vapor pressure?
Show Answer
Correct: (D)
Diethyl ether has the weakest intermolecular forces (no hydrogen-bond donation), so its molecules escape most readily into the vapor phase.
Q24A 10.0 L vessel at 298 K contains 4.00 g of He and 16.0 g of CH4. What is the partial pressure of CH4?
Show Answer
Correct: (C)
n(He) = 0.999 mol, n(CH4) = 0.998 mol, so the mole fraction of CH4 is 0.500; total pressure = 4.88 atm, giving P(CH4) = 0.500 x 4.88 atm = 2.44 atm.
Q25Which compound has the highest boiling point?
Show Answer
Correct: (A)
Ethanol can form hydrogen bonds, which are stronger than the dipole-dipole forces in dimethyl ether and fluoroethane or the London dispersion forces in propane.
🔥 Thermochemistry
Q26Calculate ΔH° for the combustion of ethane, C2H6(g) + 7/2 O2(g) → 2 CO2(g) + 3 H2O(l), given ΔHf° values (kJ/mol): CO2(g) = -394, H2O(l) = -286, C2H6(g) = -85.
Show Answer
Correct: (B)
ΔH = [2(-394) + 3(-286)] – (-85) = -1646 + 85 = -1561 kJ.
Q27When 4.50 g of NH4NO3 (80.05 g/mol) dissolves in 60.0 g of water, the temperature of the solution drops from 22.4°C to 18.9°C. What is the molar enthalpy of solution of NH4NO3? (assume the specific heat of the solution is 4.184 J/g·°C)
Show Answer
Correct: (D)
The solution loses q = (64.5 g)(4.184 J/g·°C)(-3.5°C) = -945 J, so the dissolution absorbs +945 J; 4.50 g = 0.0562 mol, giving ΔHsoln = +16.8 kJ/mol (endothermic).
Q28Calculate ΔH for 2 NO(g) + O2(g) → 2 NO2(g), given:
N2(g) + O2(g) → 2 NO(g) ΔH = +180 kJ
N2(g) + 2 O2(g) → 2 NO2(g) ΔH = +66 kJ
Show Answer
Correct: (A)
Subtracting the first equation from the second gives the target reaction with ΔH = 66 – 180 = -114 kJ.
Q29How much heat is released when 36.0 g of water vapor at 100°C condenses to liquid water at 100°C? (ΔHvap = 40.7 kJ/mol)
Show Answer
Correct: (C)
36.0 g H2O = 2.00 mol; heat released = 2.00 mol x 40.7 kJ/mol = 81.4 kJ.
Q30A 150.0 g sample of copper (specific heat 0.385 J/g·°C) absorbs 2.31 kJ of heat. If the initial temperature is 25.0°C, what is the final temperature?
Show Answer
Correct: (B)
ΔT = 2310 J / (150.0 g x 0.385 J/g·°C) = 40.0°C; final temperature = 25.0 + 40.0 = 65.0°C.
Q31Given the following thermochemical data:
C(s) + O2(g) → CO2(g), ΔH = −394 kJ
CO(g) + 1/2O2(g) → CO2(g), ΔH = −283 kJ
What is ΔH for C(s) + 1/2O2(g) → CO(g)?
Show Answer
Correct: (D)
Reverse the second equation (+283 kJ) and add to the first: −394 + 283 = −111 kJ.
⚡ Quantum Theory & Electronic Structure
Q32How many angular (planar) nodes does a 3d orbital have?
Show Answer
Correct: (C)
Number of angular nodes equals the azimuthal quantum number l, and for a d orbital l = 2.
Q33Which of the following sets of quantum numbers (n, l, m_l) is NOT allowed?
Show Answer
Correct: (A)
For l = 1, |m_l| cannot exceed 1, so m_l = −2 is forbidden; all other sets are valid.
Q34An electron in a hydrogen atom falls from n = 3 to n = 2, emitting a photon with energy 1.89 eV. What is the frequency of this photon? (h = 6.626 × 10^−34 J·s; 1 eV = 1.602 × 10^−19 J)
Show Answer
Correct: (D)
E = 1.89 × 1.602 × 10^−19 = 3.03 × 10^−19 J; ν = E/h = 4.57 × 10^14 s^−1.
Q35A metal has a work function of 4.0 eV. What is the longest wavelength of light that can still eject electrons from this metal? (hc = 1240 eV·nm)
Show Answer
Correct: (A)
Threshold: hc/λ_max = 4.0 eV, so λ_max = 1240/4.0 = 310 nm; longer wavelengths lack sufficient photon energy.
⏱ Chemical Kinetics
Q36Initial-rate data for X + Y → products:
Expt 1: [X] = 0.20 M, [Y] = 0.10 M, rate = 3.0 × 10^−4 M/s
Expt 2: [X] = 0.40 M, [Y] = 0.10 M, rate = 6.0 × 10^−4 M/s
Expt 3: [X] = 0.20 M, [Y] = 0.30 M, rate = 2.7 × 10^−3 M/s
What is the rate law?
Show Answer
Correct: (B)
Doubling [X] doubles the rate (first order in X); tripling [Y] increases the rate ninefold (second order in Y).
Q37A first-order decomposition has a half-life of 45 min. Starting with 1.00 g of reactant, how long will it take for 0.125 g to remain?
Show Answer
Correct: (A)
0.125 g is (1/2)^3 of the original, so 3 half-lives elapse: 3 × 45 = 135 min.
Q38Two reactions have the same frequency factor A at 298 K. Reaction 1 has E_a = 80 kJ/mol and reaction 2 has E_a = 100 kJ/mol. Approximately how much faster is reaction 1 than reaction 2? (R = 8.314 J/mol·K)
Show Answer
Correct: (A)
k1/k2 = e^(ΔE_a/RT) = e^(20000/(8.314 × 298)) = e^8.07 ≈ 3.2 × 10^3.
Q39Which statement best explains why reaction rates generally increase with increasing temperature?
Show Answer
Correct: (B)
Higher T shifts the Boltzmann distribution so more collisions have E ≥ E_a; the collision frequency increase is comparatively minor.
Q40A proposed mechanism is:
Step 1 (fast): A + B → C + D
Step 2 (slow): C + E → F + G
Which species is the reaction intermediate?
Show Answer
Correct: (D)
C is produced in step 1 and consumed in step 2, never appearing in the overall equation, which defines an intermediate.
Q41A plot of 1/[A] versus time for a decomposition reaction gives a straight line. What is the order of the reaction with respect to A?
Show Answer
Correct: (A)
The integrated rate law 1/[A]_t = kt + 1/[A]_0 is linear in time only for a second-order reaction.
⇆ Chemical Equilibrium
Q42For H2(g) + I2(g) ⇌ 2HI(g), the equilibrium concentrations are [H2] = 0.10 M, [I2] = 0.10 M, and [HI] = 0.50 M. What is Kc?
Show Answer
Correct: (B)
Kc = [HI]^2/([H2][I2]) = (0.50)^2/(0.10 × 0.10) = 0.25/0.010 = 25.
Q43For 2NO2(g) ⇌ N2O4(g) at equilibrium, the volume of the container is suddenly decreased at constant temperature. What happens?
Show Answer
Correct: (C)
Decreasing volume raises total pressure, favoring the side with fewer moles of gas (1 mol N2O4 vs 2 mol NO2).
Q44For which of the following reactions does Kp equal Kc at any given temperature?
Show Answer
Correct: (D)
Kp = Kc(RT)^Δn; only this reaction has Δn = 0 (2 gas mol → 2 gas mol), so Kp = Kc.
Q45For A(g) ⇌ 2B(g), Kp = 0.50. A mixture initially has P_A = 2.0 atm and P_B = 1.0 atm. Which statement is correct?
Show Answer
Correct: (C)
Qp = P_B^2/P_A = (1.0)^2/2.0 = 0.50 = Kp, so the system is at equilibrium.
Q46For the reaction 2 SO2(g) + O2(g) ⇆ 2 SO3(g), Kc = 4.6 × 10^4 at 900 K. What is the value of Kp? (R = 0.0821 L·atm/mol·K)
Show Answer
Correct: (A)
Δn = 2 – 3 = -1, so Kp = Kc(RT)^Δn = (4.6 × 10^4)/(0.0821 × 900) = 6.2 × 10^2.
Q47For PCl5(g) ⇆ PCl3(g) + Cl2(g), Kc = 0.042 at 250°C. A 5.00 L flask contains 0.100 mol PCl5, 0.200 mol PCl3, and 0.150 mol Cl2. In which direction will the reaction proceed?
Show Answer
Correct: (B)
Q = (0.0400)(0.0300)/(0.0200) = 0.060 > Kc = 0.042, so the reaction shifts left toward reactants.
🧫 Acids, Bases & Buffers
Q48What is the pH of a 0.20 M solution of pyridine, C5H5N (Kb = 1.7 × 10^-9)?
Show Answer
Correct: (A)
[OH-] = √(Kb × C) = 1.8 × 10^-5 M, so pOH = 4.73 and pH = 9.27.
Q49The first ionization of carbonic acid has Ka1 = 4.3 × 10^-7. What is the pH of a 0.10 M H2CO3 solution? (Ignore the second ionization.)
Show Answer
Correct: (D)
[H+] = √(Ka1 × C) = √(4.3 × 10^-8) = 2.1 × 10^-4 M, so pH = 3.68.
Q50A buffer contains 0.30 M acetic acid and 0.30 M sodium acetate. What is the pH after 0.10 mol of HCl is added per liter of buffer? (Ka = 1.8 × 10^-5)
Show Answer
Correct: (C)
The added H+ converts acetate to acetic acid: [HA] = 0.40 M, [A-] = 0.20 M; pH = 4.74 + log(0.20/0.40) = 4.44.
Q51A 0.10 M solution of methylamine, CH3NH2 (Kb = 4.4 × 10^-4), is titrated with 0.10 M HCl. Which statement is correct?
Show Answer
Correct: (B)
At the equivalence point only CH3NH3+ is present, which hydrolyzes to give an acidic solution (pH < 7).
Q52The Kb of ammonia is 1.8 × 10^-5. What is the Ka of the ammonium ion, NH4+?
Show Answer
Correct: (A)
Ka × Kb = Kw, so Ka = (1.0 × 10^-14)/(1.8 × 10^-5) = 5.6 × 10^-10.
Q53Consider the acids: HClO (Ka = 3.5 × 10^-8), HCN (Ka = 6.2 × 10^-10), HF (Ka = 6.8 × 10^-4), CH3COOH (Ka = 1.8 × 10^-5). Which conjugate base is the strongest?
Show Answer
Correct: (B)
The weakest acid (HCN, smallest Ka) has the strongest conjugate base (CN-).
🔋 Thermodynamics & Electrochemistry
Q54Which set of conditions gives a reaction that is spontaneous at all temperatures?
Show Answer
Correct: (A)
With ΔH negative and -TΔS negative, ΔG = ΔH – TΔS is negative at every temperature.
Q55For the reaction 2 H2O2(l) → 2 H2O(l) + O2(g), ΔH° = -196 kJ and ΔS° = +125 J/K. What is ΔG° at 298 K?
Show Answer
Correct: (D)
ΔG° = ΔH° – TΔS° = -196 kJ – (298 K)(0.125 kJ/K) = -233 kJ.
Q56For the galvanic cell Mg(s) | Mg^2+(1 M) || Ag+(1 M) | Ag(s), with E°(Mg^2+/Mg) = -2.37 V and E°(Ag+/Ag) = +0.80 V, what is the standard cell potential?
Show Answer
Correct: (C)
E°cell = E°cathode – E°anode = (+0.80 V) – (-2.37 V) = 3.17 V.
Q57How long must a current of 4.00 A flow to deposit 12.7 g of copper (molar mass 63.5 g/mol) from CuSO4(aq)?
Show Answer
Correct: (B)
12.7 g Cu = 0.200 mol, requiring 0.400 mol e- (Cu^2+ + 2e- → Cu); charge = 0.400 × 96485 = 3.86 × 10^4 C; time = 3.86 × 10^4/4.00 = 9.65 × 10^3 s.
Q58Given E° values: Mg^2+/Mg = -2.37 V, Zn^2+/Zn = -0.76 V, Cu^2+/Cu = +0.34 V, Ag+/Ag = +0.80 V. Which species is the strongest reducing agent?
Show Answer
Correct: (D)
The strongest reducing agent is the species most easily oxidized, which corresponds to the most negative reduction potential: Mg(s).
☢ Nuclear & Descriptive Chemistry
Q59Complete the nuclear equation: ^238_92U → ^234_90Th + ?
Show Answer
Correct: (A)
Mass number: 238 = 234 + 4; atomic number: 92 = 90 + 2, identifying an alpha particle, ^4_2He.
Q60Which ion is correctly matched with its formula and charge?
Show Answer
Correct: (C)
Carbonate is CO3^2-; sulfate is SO4^2-, nitrate is NO3-, and hydroxide is OH-.
Want more practice?