AP Chemistry Practice Test 18
60 original exam-style multiple-choice questions with answers & explanations — covering all nine units of the AP Chemistry course.
Note: These are original practice questions written in the AP Chemistry exam style — not reproduced College Board questions. Use them to test your readiness for the real AP Chemistry exam.
📱 How to use: Try each question first, then click “Show Answer” to check. Covers all 9 AP units: atomic structure, compound structure, intermolecular forces, reactions, kinetics, thermodynamics, equilibrium, acids & bases, and applications of thermodynamics.
⚛ Unit 1: Atomic Structure & Properties
Q1A photoelectron spectrum of an unknown element shows six peaks, in order of decreasing binding energy, with relative heights 2, 2, 6, 2, 6, and 1. Which element is it?
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Correct: (C)
The heights sum to 19 electrons and match 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1, the configuration of potassium.
Q2What is the ground-state electron configuration of a copper atom?
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Correct: (A)
Copper is an exception to the expected filling order; a filled 3d subshell with a half-filled 4s subshell is more stable.
Q3Which of the following elements has the highest electronegativity?
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Correct: (D)
Electronegativity increases across a period and decreases down a group, and Cl is the most upper-right element in this set.
Q4Chlorine consists of 75.78% Cl-35 (mass 34.969 amu) and 24.22% Cl-37 (mass 36.966 amu). What is the average atomic mass of chlorine?
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Correct: (B)
(0.7578 × 34.969) + (0.2422 × 36.966) = 26.50 + 8.95 = 35.45 amu.
Q5How many unpaired electrons are in an Fe^2+ ion in its ground state?
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Correct: (A)
Fe^2+ is [Ar] 3d^6; by Hund's rule, five d electrons occupy separate orbitals and the sixth pairs up, leaving 4 unpaired.
Q6Which alkaline earth metal has the largest atomic radius?
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Correct: (C)
Atomic radius increases down a group as new electron shells are added, and Ba is the lowest of these in group 2.
Q7What is the energy of a photon of light with a wavelength of 600. nm? Use h = 6.626 × 10^-34 J s and c = 3.00 × 10^8 m/s.
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Correct: (D)
E = hc/lambda = (6.626 × 10^-34 × 3.00 × 10^8) / (6.00 × 10^-7) = 3.31 × 10^-19 J.
🔗 Unit 2: Molecular & Ionic Compound Structure
Q8How many valence electrons are in one sulfate ion, SO4^2-?
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Correct: (B)
Sulfur contributes 6, four oxygens contribute 24, and the 2- charge adds 2, for a total of 6 + 24 + 2 = 32.
Q9In one resonance structure of the nitrate ion, NO3-, a particular oxygen atom is connected to nitrogen by a single bond and carries three lone pairs. What is the formal charge on that oxygen atom?
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Correct: (D)
Formal charge = 6 valence electrons – (6 nonbonding + 1 bonding) = -1.
Q10What is the molecular geometry of BF3?
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Correct: (A)
Boron has three bonding domains and no lone pairs, so the three fluorines lie in a plane at 120° angles.
Q11What is the hybridization of the oxygen atom in a water molecule, H2O?
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Correct: (C)
Oxygen has four electron domains (two bonds and two lone pairs), which corresponds to sp^3 hybridization.
Q12Which of the following alkali metal fluorides has the lattice energy with the smallest magnitude?
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Correct: (B)
All have the same charges, but Rb+ is the largest cation, so the ions are farthest apart and the attractions are weakest.
Q13What is the bond order of the oxygen-oxygen bond in an O2 molecule, based on its Lewis structure?
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Correct: (D)
The Lewis structure of O2 contains a double bond, so the bond order is 2.
Q14Which central atom can accommodate an expanded octet in a stable compound?
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Correct: (A)
Phosphorus is in period 3, so it can hold more than eight valence electrons, as in PCl5 with five bonds; C, N, and O in period 2 cannot.
💧 Unit 3: Intermolecular Forces & Properties
Q15Which type of intermolecular force is present in every substance, regardless of its composition?
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Correct: (C)
Temporary dipoles arise from electron motion in all atoms and molecules, so London dispersion forces occur in all substances.
Q16Ethanol, CH3CH2OH, and dimethyl ether, CH3OCH3, have the same molar mass, yet ethanol boils at 78 C while dimethyl ether boils at -24 C. Which statement best explains this difference?
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Correct: (B)
Ethanol has an O-H group, so its molecules hydrogen bond and need more thermal energy to separate; the ether has no O-H bond to donate.
Q17Which of the following liquids has the highest vapor pressure at 25 C?
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Correct: (D)
Diethyl ether has only weak dipole and dispersion forces and no hydrogen bond donation, so its molecules escape into the vapor most easily.
Q18How much heat is absorbed when 18.0 g of water is vaporized at its normal boiling point? The molar enthalpy of vaporization of water is 40.7 kJ/mol.
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Correct: (A)
18.0 g of H2O is 1.00 mol (18.0 / 18.0), so the heat absorbed is 1.00 mol × 40.7 kJ/mol = 40.7 kJ.
Q19A solid conducts electricity when molten but not in the solid state, and it shatters when struck. Which substance fits this description?
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Correct: (B)
Ionic solids like KCl cannot conduct when solid because the ions are locked in place, but the ions move freely and carry charge when molten.
Q20A solution is made by dissolving 20.0 g of sugar in 80.0 g of water. What is the mass percent of sugar in the solution?
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Correct: (C)
Mass percent = 20.0 g / (20.0 g + 80.0 g) × 100% = 20.0%.
⚖ Unit 4: Chemical Reactions
Q21When the combustion equation C2H6 + O2 → CO2 + H2O is balanced with the smallest whole-number coefficients, what is the coefficient of O2?
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Correct: (C)
2 C2H6 + 7 O2 → 4 CO2 + 6 H2O balances C, H, and O, so the O2 coefficient is 7.
Q22In the reaction Mg(s) + Cu^2+(aq) → Mg^2+(aq) + Cu(s), which element is reduced?
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Correct: (A)
Cu^2+ gains electrons to form Cu(s); its oxidation number falls from +2 to 0, so copper is reduced.
Q23Aqueous HCl and aqueous NaOH are mixed. What is the net ionic equation for the neutralization?
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Correct: (D)
HCl and NaOH are strong electrolytes; Na^+ and Cl^- are spectators, leaving H^+ + OH^- → H2O.
Q24Calcium carbonate decomposes by CaCO3(s) → CaO(s) + CO2(g). How many grams of CO2 are produced when 10.0 g of CaCO3 (molar mass 100. g/mol) decomposes completely? The molar mass of CO2 is 44.0 g/mol.
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Correct: (B)
10.0 g CaCO3 = 0.100 mol, which gives 0.100 mol CO2; 0.100 mol × 44.0 g/mol = 4.40 g.
Q25Hydrogen and oxygen react by 2 H2(g) + O2(g) → 2 H2O(l). A mixture contains 4.0 mol of H2 and 1.0 mol of O2. Which reactant is limiting, and how many moles of H2O can form?
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Correct: (C)
4.0 mol H2 would require 2.0 mol O2, so O2 is limiting; 1.0 mol O2 × (2 mol H2O / 1 mol O2) = 2.0 mol H2O.
Q26A synthesis has a theoretical yield of 50.0 g. If the percent yield is 90.0%, what is the actual yield?
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Correct: (A)
Actual yield = 0.900 × 50.0 g = 45.0 g.
Q27How many milliliters of 0.200 M NaOH(aq) are needed to neutralize 50.00 mL of 0.100 M HCl(aq)?
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Correct: (D)
Moles HCl = 0.100 M × 0.05000 L = 0.00500 mol; volume NaOH = 0.00500 mol / 0.200 M = 0.02500 L = 25.00 mL.
⏱ Unit 5: Kinetics
Q28For the reaction X + Y → products, the following data were collected: Experiment 1: [X] = 0.10 M, [Y] = 0.10 M, rate = 0.030 M/s; Experiment 2: [X] = 0.20 M, [Y] = 0.10 M, rate = 0.120 M/s; Experiment 3: [X] = 0.20 M, [Y] = 0.20 M, rate = 0.120 M/s. What is the rate law?
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Correct: (B)
Doubling [X] quadruples the rate (second order in X); doubling [Y] has no effect (zero order in Y).
Q29A reaction is first order in A. When [A] = 0.30 M, the rate is 0.060 M/s. What is the value of the rate constant k?
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Correct: (C)
For rate = k[A], k = 0.060 M/s / 0.30 M = 0.20 s^-1.
Q30Ozone decomposes by the mechanism: Step 1: O3(g) → O2(g) + O(g); Step 2: O(g) + O3(g) → 2 O2(g). Which species is an intermediate in this mechanism?
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Correct: (A)
O(g) is produced in Step 1 and consumed in Step 2 and does not appear in the overall equation, so it is an intermediate.
Q31According to collision theory, why does increasing the concentration of a reactant usually increase the reaction rate?
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Correct: (D)
More particles per unit volume collide more often, so more effective collisions occur per unit time.
Q32On an energy diagram for a reaction, the reactants are at 20 kJ/mol and the top of the energy barrier is at 120 kJ/mol. What is the activation energy for the forward reaction?
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Correct: (B)
Ea = energy of the transition state − energy of reactants = 120 − 20 = 100 kJ/mol.
Q33Which statement about a catalyst at equilibrium is correct?
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Correct: (C)
A catalyst lowers the barrier in both directions equally; it speeds attainment of equilibrium without changing its composition or K.
Q34A first-order reaction has [A] = 1.00 M initially. After 30. min, [A] = 0.25 M. What is the rate constant k?
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Correct: (A)
1.00 → 0.50 → 0.25 M is two half-lives in 30. min, so t1/2 = 15 min and k = 0.693 / 15 min = 0.046 min^-1.
🔥 Unit 6: Thermodynamics
Q35Given: (1) N2(g) + O2(g) → 2 NO(g), ΔH = +180 kJ/mol; (2) 2 NO(g) + O2(g) → 2 NO2(g), ΔH = −112 kJ/mol. What is ΔH for N2(g) + 2 O2(g) → 2 NO2(g)?
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Correct: (D)
By Hess's law, adding the two steps gives ΔH = +180 + (−112) = +68 kJ/mol.
Q36How much heat is absorbed when 150. g of water is heated by 5.0°C? The specific heat capacity of water is 4.184 J/(g·°C).
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Correct: (B)
q = mcΔT = 150. g × 4.184 J/(g·°C) × 5.0°C = 3138 J = 3.14 kJ.
Q37Using bond enthalpies H−H = 436 kJ/mol, Br−Br = 193 kJ/mol, and H−Br = 366 kJ/mol, what is ΔH for H2(g) + Br2(g) → 2 HBr(g)?
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Correct: (C)
ΔH = bonds broken − bonds formed = (436 + 193) − 2(366) = 629 − 732 = −103 kJ/mol.
Q38Which process is endothermic under standard conditions?
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Correct: (A)
A positive ΔH means heat is absorbed from the surroundings, which defines an endothermic process; the others have negative ΔH values.
Q39For which reaction is ΔS expected to be negative?
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Correct: (D)
Three moles of gas are converted to a liquid, a large decrease in disorder and in gas moles, so ΔS < 0.
Q40Given the standard enthalpy of formation of H2O(l) is ΔHf = −286 kJ/mol, and ΔHf = 0 for H2(g) and O2(g), what is ΔH for 2 H2(g) + O2(g) → 2 H2O(l)?
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Correct: (B)
ΔH = 2(−286 kJ/mol) − 0 = −572 kJ/mol.
Q41A reaction has ΔH > 0 and ΔS < 0. Which statement about its spontaneity is correct?
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Correct: (C)
ΔG = ΔH − TΔS is positive at all temperatures when ΔH is positive and ΔS is negative, so the reaction is never spontaneous.
⇆ Unit 7: Equilibrium
Q42A mixture initially contains 0.50 M CO(g) and 0.50 M H2O(g). At equilibrium, [CO2] = 0.30 M for the reaction CO(g) + H2O(g) ⇌ CO2(g) + H2(g). What is the value of Kc?
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Correct: (C)
Forming 0.30 M CO2 also forms 0.30 M H2 and leaves 0.20 M each of CO and H2O. Kc = (0.30)(0.30) / (0.20)(0.20) = 2.25.
Q43For the equilibrium N2O4(g) ⇌ 2 NO2(g), the volume of the container is suddenly decreased, increasing the pressure. Which statement describes the immediate shift?
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Correct: (A)
Compression favors the side with fewer moles of gas. The left side has 1 mol of gas versus 2 mol on the right, so the equilibrium shifts left; Kc is unchanged at constant temperature.
Q44For the reaction SO2Cl2(g) ⇌ SO2(g) + Cl2(g), Kc = 0.045 at 600 K. What is the value of Kp at 600 K? (R = 0.0821 L·atm/mol·K)
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Correct: (D)
Δn = 2 – 1 = +1, so Kp = Kc(RT)^1 = 0.045 × (0.0821 × 600) = 0.045 × 49.2 = 2.2.
Q45The molar solubility of Ag2CrO4 in pure water is 6.5 × 10^-5 M. What is the Ksp of Ag2CrO4?
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Correct: (B)
For Ag2CrO4(s) ⇌ 2 Ag^+(aq) + CrO4^2-(aq), Ksp = (2s)^2(s) = 4s^3 = 4(6.5 × 10^-5)^3 = 1.1 × 10^-12.
Q46A solution contains [Ag^+] = 2.0 × 10^-4 M and [Cl^-] = 3.0 × 10^-3 M. The Ksp of AgCl is 1.8 × 10^-10. Which prediction is correct?
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Correct: (C)
Q = (2.0 × 10^-4)(3.0 × 10^-3) = 6.0 × 10^-7, which is much greater than Ksp = 1.8 × 10^-10, so AgCl precipitates.
Q47The Ksp of Mg(OH)2 is 5.6 × 10^-12. What is the molar solubility of Mg(OH)2 in a 0.10 M MgCl2 solution?
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Correct: (A)
With [Mg^2+] fixed near 0.10 M by the common ion, Ksp = (0.10)(2s)^2 = 0.40s^2, so s^2 = 1.4 × 10^-11 and s = 3.7 × 10^-6 M.
Q48COCl2(g) is placed in a container at an initial concentration of 0.40 M. For COCl2(g) ⇌ CO(g) + Cl2(g), Kc = 0.020. What is the equilibrium concentration of CO?
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Correct: (D)
Setting x^2 / (0.40 – x) = 0.020 gives x^2 + 0.020x – 0.0080 = 0, so x = 0.080 M, and [CO] = x.
🧫 Unit 8: Acids & Bases
Q49What is the pH of a 0.15 M NH3(aq) solution at 25°C? (Kb for NH3 = 1.8 × 10^-5)
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Correct: (B)
[OH^-] = sqrt(Kb × C) = sqrt(1.8 × 10^-5 × 0.15) = 1.6 × 10^-3 M, so pOH = 2.78 and pH = 14.00 – 2.78 = 11.22.
Q50What is the pH of a 0.0040 M Ba(OH)2(aq) solution at 25°C?
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Correct: (D)
Each Ba(OH)2 unit releases two OH^-, so [OH^-] = 0.0080 M. pOH = -log(0.0080) = 2.10 and pH = 14.00 – 2.10 = 11.90.
Q51A buffer contains 0.20 M NH3 and 0.30 M NH4Cl. The pKa of NH4^+ is 9.25. What is the pH of the buffer?
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Correct: (A)
pH = pKa + log([base]/[acid]) = 9.25 + log(0.20/0.30) = 9.25 – 0.18 = 9.07.
Q52Propanoic acid has Ka = 1.3 × 10^-5. A sample of propanoic acid is titrated with NaOH, and the titration is stopped at the half-equivalence point. What is the pH at this point?
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Correct: (C)
At the half-equivalence point [HA] = [A^-], so pH = pKa = -log(1.3 × 10^-5) = 4.89.
Q53A 0.30 M solution of nitrous acid, HNO2, has a pH of 2.10 at 25°C. What is the Ka of HNO2?
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Correct: (B)
[H^+] = 10^-2.10 = 7.9 × 10^-3 M. Ka = (7.9 × 10^-3)^2 / (0.30 – 0.0079) = 6.3 × 10^-5 / 0.292 = 2.2 × 10^-4.
Q54Phosphoric acid has Ka1 = 7.5 × 10^-3 (pKa1 = 2.12) and Ka2 = 6.2 × 10^-8 (pKa2 = 7.21). A solution of NaH2PO4 contains the amphiprotic ion H2PO4^-. Its pH is closest to
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Correct: (D)
For an amphiprotic species, pH approx equals the average of the two surrounding pKa values: (2.12 + 7.21) / 2 = 4.67.
🔋 Unit 9: Applications of Thermodynamics
Q55Using standard free energies of formation, ΔGf° of CH4(g) = -50.8 kJ/mol, CO2(g) = -394.4 kJ/mol, and H2O(l) = -237.2 kJ/mol, what is ΔG° for CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)?
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Correct: (A)
ΔG° = [(-394.4) + 2(-237.2)] – [(-50.8) + 2(0)] = -868.8 + 50.8 = -818 kJ/mol. O2 is an element in its standard state, so its ΔGf° is zero.
Q56Standard molar entropies are S°(H2) = 131 J/mol·K, S°(O2) = 205 J/mol·K, and S°(H2O(l)) = 69.9 J/mol·K. What is ΔS° for 2 H2(g) + O2(g) → 2 H2O(l)?
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Correct: (C)
ΔS° = 2(69.9) – [2(131) + 205] = 139.8 – 467 = -327 J/mol·K, consistent with three moles of gas forming a liquid.
Q57A galvanic cell uses the reaction Ni(s) + Pb^2+(aq) → Ni^2+(aq) + Pb(s). Given E° for Pb^2+/Pb is -0.13 V and E° for Ni^2+/Ni is -0.25 V, what is E°cell?
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Correct: (B)
Nickel is oxidized (anode) and Pb^2+ is reduced (cathode), so E°cell = -0.13 V – (-0.25 V) = +0.12 V.
Q58Silver is deposited from an AgNO3 solution by the half-reaction Ag^+ + e^- → Ag. How long must a current of 1.50 A run to deposit 2.16 g of Ag? (Faraday constant = 96,500 C/mol e^-; molar mass of Ag = 107.9 g/mol)
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Correct: (D)
Moles of Ag = 2.16 / 107.9 = 0.0200 mol, requiring 0.0200 mol e^-, or 0.0200 × 96,500 = 1930 C. Time = 1930 C / 1.50 A = 1290 s.
Q59At 298 K, a reaction has an equilibrium constant of K = 1.0 × 10^-2. Which statement about the reaction under standard conditions is correct? (R = 8.314 J/mol·K)
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Correct: (A)
ΔG° = -RT ln K = -(8.314)(298) ln(1.0 × 10^-2) = +11,400 J/mol = +11.4 kJ/mol. A positive ΔG° (K less than 1) means the reaction is nonspontaneous under standard conditions.
Q60A galvanic cell operates with E°cell = +0.50 V and transfers 2 mol of electrons per mole of reaction. What is ΔG° for the reaction? (Faraday constant = 96,500 C/mol e^-)
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Correct: (C)
ΔG° = -nFE° = -(2)(96,500)(0.50) = -96,500 J/mol = -97 kJ/mol.
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