ACS General Chemistry Practice Test 20
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
Q1Which element has the smallest atomic radius?
Show Answer
Correct: (D)
Atomic radius decreases across a period and increases down a group; Br (period 4, group 17) is the smallest of these due to high effective nuclear charge.
Q2Arrange the following ions in order of increasing radius: S2-, Cl-, K+, Ca2+.
Show Answer
Correct: (D)
These are isoelectronic (18 electrons each); radius decreases as nuclear charge increases: Ca2+ (Z=20) < K+ (Z=19) < Cl- (Z=17) < S2- (Z=16).
Q3Which has the lowest second ionization energy?
Show Answer
Correct: (B)
Removing a second electron from Mg+ gives the stable noble gas configuration [Ne]; Na+ already has noble gas configuration so its second IE is very high.
Q4Which element has the most exothermic electron affinity?
Show Answer
Correct: (D)
Among chalcogens, S has the highest electron affinity; O is smaller with greater electron repulsion, and EA decreases down the group after S.
Q5The electron configuration [Ar] 4s1 3d5 corresponds to:
Show Answer
Correct: (A)
Chromium's ground state is the exceptional [Ar] 4s1 3d5 (half-filled d subshell is more stable than 4s2 3d4).
Q6Which of the following is NOT a periodic trend?
Show Answer
Correct: (D)
Neutron number is not a periodic trend; the others are well-established periodic properties.
🔬 Chemical Bonding & Molecular Geometry
Q7What is the molecular geometry of XeF4?
Show Answer
Correct: (D)
XeF4 has 6 electron domains (4 bonds + 2 lone pairs); lone pairs occupy axial positions giving square planar molecular geometry.
Q8Which molecule has sp2 hybridization on the central atom?
Show Answer
Correct: (B)
BF3 has 3 electron domains (trigonal planar), requiring sp2 hybridization; CH4 is sp3, NH3 is sp3, H2O is sp3.
Q9Which of the following bonds is most polar?
Show Answer
Correct: (A)
Bond polarity increases with electronegativity difference; F (4.0) – H (2.1) = 1.9 is the largest difference listed.
Q10Which substance exhibits dipole-dipole interactions but NOT hydrogen bonding?
Show Answer
Correct: (C)
CH3Cl is polar but has no H bonded to N, O, or F; H2O, HF, and NH3 all exhibit hydrogen bonding.
Q11What is the approximate bond angle in NH3?
Show Answer
Correct: (B)
NH3 is trigonal pyramidal; the lone pair compresses the H-N-H angle from 109.5 degrees to approximately 107 degrees.
Q12Which has the longest bond length?
Show Answer
Correct: (B)
Bond length increases with atomic size; I is the largest halogen listed, so I-I is the longest bond.
Q13Which molecule violates the octet rule by having fewer than 8 electrons around the central atom?
Show Answer
Correct: (C)
Boron in BF3 has only 6 valence electrons (electron-deficient); SF6 and PCl5 have expanded octets.
🧪 Stoichiometry & Aqueous Reactions
Q14What mass of CO2 is produced when 10.0 g of CaCO3 decomposes? (CaCO3 → CaO + CO2; Ca = 40.1, C = 12.0, O = 16.0 g/mol)
Show Answer
Correct: (D)
10.0 g CaCO3 = 0.100 mol; produces 0.100 mol CO2 = 0.100 × 44.0 = 4.40 g.
Q15What is the molar mass of Al2(SO4)3? (Al = 27.0, S = 32.1, O = 16.0 g/mol)
Show Answer
Correct: (C)
2(27.0) + 3(32.1) + 12(16.0) = 54.0 + 96.3 + 192.0 = 342.3 g/mol (closest to 342.1).
Q16A 5.00 g sample of a compound contains 2.00 g C, 0.336 g H, and 2.66 g O. What is the empirical formula?
Show Answer
Correct: (D)
C: 2.00/12.0 = 0.167 mol; H: 0.336/1.008 = 0.333 mol; O: 2.66/16.0 = 0.166 mol; ratio 1:2:1 gives CH2O.
Q17How many grams of NaCl are needed to prepare 500 mL of 0.200 M solution? (NaCl = 58.5 g/mol)
Show Answer
Correct: (B)
Moles needed = 0.500 L × 0.200 M = 0.100 mol; mass = 0.100 × 58.5 = 5.85 g.
Q18The reaction N2 + 3 H2 → 2 NH3 produces 8.50 g NH3 from 14.0 g N2 with excess H2. What is the percent yield? (N = 14.0, H = 1.0 g/mol)
Show Answer
Correct: (D)
14.0 g N2 = 0.500 mol; theoretical NH3 = 1.00 mol = 17.0 g; percent yield = (8.50/17.0) × 100% = 50.0%.
Q19Which of the following is the strongest electrolyte?
Show Answer
Correct: (C)
KOH is a strong base (soluble hydroxide) that dissociates completely; HC2H3O2 is weak, H2O barely ionizes, sugar is a nonelectrolyte.
Q20What is the oxidation number of N in NO3-?
Show Answer
Correct: (C)
O is -2; x + 3(-2) = -1; x = +5.
🌀 Gases & Intermolecular Forces
Q21A sample of gas at 3.00 atm and 300 K occupies 4.00 L. What is the pressure if the volume is reduced to 1.00 L at constant temperature?
Show Answer
Correct: (D)
Boyle's law: P2 = P1V1/V2 = (3.00 atm × 4.00 L)/1.00 L = 12.0 atm.
Q22How many moles of gas are in a 5.00 L container at 2.00 atm and 273 K? (R = 0.0821 L·atm/mol·K)
Show Answer
Correct: (B)
n = PV/RT = (2.00 × 5.00)/(0.0821 × 273) = 10.0/22.4 = 0.446 mol.
Q23Two gases, A (molar mass 16 g/mol) and B (molar mass 64 g/mol), effuse through a pinhole. How much faster does A effuse than B?
Show Answer
Correct: (A)
Graham's law: rateA/rateB = √(MB/MA) = √(64/16) = √4 = 2.
Q24A gas mixture contains 2.0 g H2 and 8.0 g O2. What is the mole fraction of H2? (H = 1.0, O = 16.0 g/mol)
Show Answer
Correct: (A)
Moles H2 = 2.0/2.0 = 1.0 mol; moles O2 = 8.0/32.0 = 0.25 mol; X(H2) = 1.0/1.25 = 0.80.
Q25Which intermolecular force is present in all molecules?
Show Answer
Correct: (C)
London dispersion forces arise from temporary dipoles and exist in all molecules; the others require specific structural features.
🔥 Thermochemistry
Q26A 25.0 g sample of aluminum (c = 0.902 J/g·°C) cools from 100.0°C to 25.0°C. How much heat is released?
Show Answer
Correct: (A)
q = mcΔT = 25.0 × 0.902 × 75.0 = 1,691 J (heat released, so -1,690 J).
Q27Which process is endothermic?
Show Answer
Correct: (D)
Melting requires energy input to break intermolecular forces; freezing, condensation, and combustion all release energy.
Q28Given ΔHf° values: CO2(g) = -393.5 kJ/mol, H2O(l) = -285.8 kJ/mol, CH4(g) = -74.8 kJ/mol. What is ΔH° for CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)?
Show Answer
Correct: (A)
ΔH° = [(-393.5) + 2(-285.8)] – [(-74.8) + 0] = -965.1 + 74.8 = -890.3 kJ.
Q29A bomb calorimeter measures:
Show Answer
Correct: (B)
Bomb calorimeters are rigid sealed vessels (constant volume), measuring ΔE (qV); coffee-cup calorimeters measure qP (ΔH).
Q30Which has the lowest standard molar entropy?
Show Answer
Correct: (A)
Diamond has a rigid, highly ordered crystal lattice with very low entropy; gases and liquids have much higher entropy.
Q31The specific heat of copper is 0.385 J/g·°C. How many grams of copper can be heated from 20.0°C to 80.0°C using 1,000 J?
Show Answer
Correct: (D)
m = q/(cΔT) = 1000/(0.385 × 60.0) = 43.3 g.
⚡ Quantum Theory & Electronic Structure
Q32What is the energy of a photon with wavelength 500 nm? (h = 6.626 × 10-34 J·s, c = 3.00 × 108 m/s)
Show Answer
Correct: (A)
E = hc/λ = (6.626 × 10-34 × 3.00 × 108)/(500 × 10-9) = 3.98 × 10-19 J.
Q33Which set of quantum numbers is NOT allowed?
Show Answer
Correct: (D)
The azimuthal quantum number l must be less than n; l = 2 is not allowed when n = 2.
Q34The maximum number of electrons in the n = 4 shell is:
Show Answer
Correct: (C)
Maximum electrons = 2n2 = 2(16) = 32.
Q35Which transition in the hydrogen atom corresponds to absorption of the highest energy photon?
Show Answer
Correct: (A)
Absorption from n=1 requires the most energy because the n=1 level is lowest; larger energy gaps occur for transitions involving n=1.
⏱ Chemical Kinetics
Q36The rate law for 2 A + B → products is rate = k[A][B]2. If [A] is doubled and [B] is tripled, the rate increases by:
Show Answer
Correct: (C)
Rate factor = (2)1 × (3)2 = 2 × 9 = 18.
Q37A reaction has the rate law rate = k[A]0. What happens to the rate if [A] is doubled?
Show Answer
Correct: (D)
Zero order means rate is independent of [A]; changing concentration has no effect on rate.
Q38Which statement about the Arrhenius equation is correct?
Show Answer
Correct: (A)
k = Ae-Ea/RT; as T increases, the exponent becomes less negative and k increases exponentially.
Q39The half-life of a second-order reaction depends on:
Show Answer
Correct: (B)
For second order, t1/2 = 1/(k[A]0), depending on both k and initial concentration (unlike first order).
Q40Which is the best definition of activation energy?
Show Answer
Correct: (B)
Ea is the minimum kinetic energy molecules must have for collisions to result in reaction (energy barrier).
Q41For the elementary step A + 2 B → products, the rate law is:
Show Answer
Correct: (B)
For an elementary step, the rate law follows the stoichiometry; molecularity of 3 (1 A + 2 B) gives rate = k[A][B]2.
⇆ Chemical Equilibrium
Q42For the reaction 2 A(g) ⇌ B(g) + C(g), Kp = 1.5 × 102. If the partial pressures are PA = 0.10 atm, PB = 0.50 atm, PC = 0.30 atm, which way will the reaction shift?
Show Answer
Correct: (D)
Qp = (PB × PC)/(PA)2 = (0.50 × 0.30)/(0.10)2 = 15; since Qp < Kp (15 < 150), the reaction shifts forward toward products to reach equilibrium.
Q43The reaction CO(g) + H2O(g) ⇌ CO2(g) + H2(g) has Kc = 1.0 at 1000 K. If [CO] = [H2O] = [CO2] = [H2] = 1.0 M, the system is:
Show Answer
Correct: (A)
Q = (1.0 × 1.0)/(1.0 × 1.0) = 1.0 = Kc, so the system is at equilibrium.
Q44Which change will increase the value of K for an endothermic reaction?
Show Answer
Correct: (C)
For endothermic reactions (ΔH > 0), increasing T shifts equilibrium toward products, increasing K (van't Hoff equation).
Q45For the equilibrium PCl5(g) ⇌ PCl3(g) + Cl2(g), what happens when Cl2 is added?
Show Answer
Correct: (B)
Adding product (Cl2) shifts equilibrium left toward reactants by Le Chatelier's principle.
Q46A catalyst added to an equilibrium mixture:
Show Answer
Correct: (B)
Catalysts lower Ea for both directions equally, speeding attainment of equilibrium without changing K or position.
Q47For N2(g) + O2(g) ⇌ 2 NO(g) (ΔH > 0), which change increases NO at equilibrium?
Show Answer
Correct: (B)
The reaction is endothermic; increasing temperature shifts equilibrium right (toward products), increasing NO.
🧫 Acids, Bases & Buffers
Q48Which is the conjugate base of H2PO4-?
Show Answer
Correct: (B)
Removing one H+ from H2PO4- gives HPO42- (conjugate base differs by one proton).
Q49What is the [OH-] in a solution with pH = 4.0? (Kw = 1.0 × 10-14)
Show Answer
Correct: (B)
pOH = 14 – pH = 10; [OH-] = 10-10 M.
Q50Which salt produces a basic solution when dissolved in water?
Show Answer
Correct: (C)
CH3COO- is the conjugate base of a weak acid and hydrolyzes to produce OH-; NaCl and KNO3 are neutral, NH4Cl is acidic.
Q51The pKa of HF is 3.17. What is the Ka?
Show Answer
Correct: (A)
Ka = 10-pKa = 10-3.17 = 6.8 × 10-4.
Q52What is the pH of 0.100 M NaOH?
Show Answer
Correct: (D)
[OH-] = 0.100 M; pOH = 1.00; pH = 14.00 – 1.00 = 13.00.
Q53Which titration has pH = 7 at the equivalence point?
Show Answer
Correct: (A)
Strong acid-strong base titration produces a neutral salt (no hydrolysis), giving pH = 7 at equivalence.
🔋 Thermodynamics & Electrochemistry
Q54Which process has a negative ΔS?
Show Answer
Correct: (C)
Combining a solid and gas to form an ordered solid decreases disorder (ΔS < 0); melting, vaporization, and dissolving increase disorder.
Q55For the reaction 2 H2(g) + O2(g) → 2 H2O(l), ΔH = -572 kJ and ΔS = -327 J/K. Is this spontaneous at 298 K?
Show Answer
Correct: (A)
ΔG = ΔH – TΔS = -572,000 – (298)(-327) = -572,000 + 97,446 = -474,554 J < 0, so spontaneous.
Q56In an electrolytic cell:
Show Answer
Correct: (C)
Electrolytic cells use external electrical energy to drive nonspontaneous reactions (ΔG > 0); the anode is negative and cathode is positive (opposite of galvanic).
Q57Which metal is most easily oxidized? (E°: Li+/Li = -3.04 V, Na+/Na = -2.71 V, Mg2+/Mg = -2.37 V, Al3+/Al = -1.66 V)
Show Answer
Correct: (A)
The most easily oxidized species has the most negative reduction potential (strongest reducing agent); Li (-3.04 V) is the strongest reductant.
Q58How many moles of electrons are needed to reduce 1.0 mol of Al3+ to Al(s)?
Show Answer
Correct: (C)
Al3+ + 3 e- → Al(s); 3 moles of electrons per mole of Al3+ reduced.
☢ Nuclear & Descriptive Chemistry
Q59Carbon-14 dating is used for:
Show Answer
Correct: (B)
14C has a half-life of 5,730 years, making it useful for dating organic artifacts up to about 50,000 years old.
Q60Which is a common use of helium?
Show Answer
Correct: (B)
Helium's inertness makes it ideal for providing an oxygen-free atmosphere in welding; it is also used in balloons and as a coolant.
Want more practice?