ACS General Chemistry Practice Test 19

Correct: (A)

Atomic radius decreases across a period due to increasing effective nuclear charge; Na is leftmost in period 3 and has the largest radius of the four.

Correct: (A)

These ions are isoelectronic (all have 10 electrons); radius decreases with increasing nuclear charge, so O2- (Z = 8) is largest.

Correct: (C)

Ionization energy generally increases across a period; N has a half-filled 2p subshell (2p3), giving it extra stability and a higher IE than O.

Correct: (B)

Although F is more electronegative, Cl has the highest electron affinity because the small F atom experiences greater electron-electron repulsion when gaining an electron.

Correct: (D)

The ground state of Mg is 1s2 2s2 2p6 3s2; promoting a 3s electron to 3p gives an excited state configuration.

Correct: (D)

Elements in the same group have similar valence electron configurations; Li/Na (group 1) and Mg/Ca (group 2) are both same-group pairs.

Correct: (B)

CO2 has two electron domains around carbon with no lone pairs (AX2), giving linear geometry; H2O and SO2 are bent, NH3 is trigonal pyramidal.

Correct: (D)

SF4 has 5 electron domains (4 bonds + 1 lone pair), requiring sp3d hybridization for trigonal bipyramidal electron geometry.

Correct: (C)

CHCl3 has a tetrahedral arrangement with different substituents, creating a net dipole; CCl4, CO2, and BCl3 are symmetric with dipoles canceling.

Correct: (D)

NH3 has N-H bonds where N is highly electronegative and small, creating strong hydrogen bonds; S and P are larger and less electronegative.

Correct: (A)

Trigonal planar geometry (AX3) has three electron domains arranged to minimize repulsion, giving 120 degree bond angles.

Correct: (C)

Bond length decreases with increasing bond order; triple bonds are shortest, followed by double, then single bonds.

Correct: (C)

P is in period 3 and can accommodate more than 8 electrons using d orbitals; PCl5 has 10 electrons around phosphorus.

Correct: (C)

C3H8 + 5 O2 → 3 CO2 + 4 H2O; 2.0 mol C3H8 × (5 mol O2/1 mol C3H8) = 10.0 mol O2.

Correct: (C)

Molar mass MgSO4 = 24.3 + 32.1 + 64.0 = 120.4 g/mol; %O = (64.0/120.4) × 100% = 53.3%.

Correct: (A)

40.0g C = 3.33 mol, 6.7g H = 6.7 mol, 53.3g O = 3.33 mol; ratio 1:2:1 gives CH2O.

Correct: (A)

Moles H2SO4 = 0.0250 L × 0.20 M = 0.0050 mol; H2SO4 is diprotic so 0.010 mol OH- needed; V = 0.010 mol/0.50 M = 0.020 L = 20.0 mL.

Correct: (C)

5.4 g Al = 0.20 mol; 0.20 mol Al × (3 mol Cu/2 mol Al) = 0.30 mol Cu; 0.30 × 63.5 = 19.1 g.

Correct: (C)

NaCl is a soluble ionic compound that dissociates completely in water; CH3COOH and NH3 are weak electrolytes, C6H12O6 is a nonelectrolyte.

Correct: (B)

K is +1, O is -2; 2(+1) + 2x + 7(-2) = 0; 2x = +12; x = +6.

Correct: (D)

Boyle's law (P1V1 = P2V2 at constant T): V2 = (1.00 atm × 2.50 L)/2.00 atm = 1.25 L.

Correct: (A)

Density = molar mass/molar volume = 28.0 g/mol / 22.4 L/mol = 1.25 g/L.

Correct: (D)

Graham's law: effusion rate is inversely proportional to the square root of molar mass; H2 (2.0 g/mol) is lightest and effuses fastest.

Correct: (B)

Mole fraction of Ne = 0.20/1.00 = 0.20; P(Ne) = 0.20 × 2.0 atm = 0.40 atm.

Correct: (D)

All are nonpolar with only London dispersion forces; larger molecules have stronger dispersion forces and higher boiling points.

Correct: (A)

q = mcΔT = 50.0 g × 4.184 J/g·°C × 60.0°C = 12,550 J.

Correct: (B)

8.0 g CH4 = 0.50 mol; 0.50 mol × 890 kJ/mol = 445 kJ released.

Correct: (C)

Combustion of hydrogen forms strong O-H bonds, releasing energy; the others require energy input (endothermic).

Correct: (D)

Hess's law: subtract the second reaction from the first; ΔH = -393.5 – (-283.0) = -110.5 kJ.

Correct: (A)

Heat lost by metal = heat gained by water; 50.0 × c × 71.5 = 100.0 × 4.184 × 3.5; c = 0.410 J/g·°C.

Correct: (B)

Gases have much higher entropy than liquids or solids due to greater molecular disorder and available microstates.

Correct: (D)

λ = c/ν = (3.0 × 108 m/s)/(5.0 × 1014 Hz) = 6.0 × 10-7 m (600 nm, orange light).

Correct: (B)

The azimuthal quantum number l determines orbital shape (l = 0 s-spherical, l = 1 p-dumbbell, l = 2 d, l = 3 f).

Correct: (C)

Number of orbitals = n2 = 32 = 9 (one 3s, three 3p, five 3d orbitals).

Correct: (A)

Energy difference is greatest for transitions to n = 1 (Lyman series); n = 2 → n = 1 releases more energy than any Balmer transition.

Correct: (C)

Overall order = sum of exponents = 2 + 1 = 3 (third order overall).

Correct: (D)

Rate ∝ [A]n; 4 = 2n gives n = 2, so the reaction is second order in that reactant.

Correct: (D)

Higher temperature increases molecular speed and collision energy; higher concentration increases collision frequency; both increase effective collisions.

Correct: (D)

Catalysts lower Ea by providing a different mechanism; they do not change ΔG, K, or equilibrium position.

Correct: (C)

60.0 min = 3 half-lives; fraction remaining = (1/2)3 = 1/8.

Correct: (A)

From the Arrhenius equation k = Ae-Ea/RT, lowering Ea increases k exponentially; a 20 kJ/mol decrease at room temperature increases the rate by roughly 3000-fold.

Correct: (C)

Reversing a reaction inverts K; Kc(reverse) = 1/Kc(forward) = 1/0.50 = 2.0.

Correct: (B)

Decreasing volume increases pressure; the equilibrium shifts toward fewer moles of gas (3 mol → 2 mol, right) to relieve the pressure.

Correct: (D)

K much less than 1 means the equilibrium lies far to the left; reactants predominate at equilibrium.

Correct: (A)

Q = [B]/[A] = 0.60/0.40 = 1.5; since Q < K (1.5 < 2.5), the reaction shifts forward to reach equilibrium.

Correct: (A)

At constant volume, adding inert gas does not change partial pressures or concentrations of reactants/products, so no shift occurs.

Correct: (B)

By Le Chatelier's principle, adding heat to an exothermic reaction shifts the equilibrium toward reactants (left), decreasing K.

Correct: (A)

HCl dissociates completely in water; HF, CH3COOH, and H2CO3 are all weak acids that only partially ionize.

Correct: (B)

HCl is a strong acid, so [H+] = 0.010 M; pH = -log(0.010) = 2.0.

Correct: (B)

NaOH is a strong base giving [OH-] = 0.10 M (pH = 13); HCl gives pH = 1, CH3COOH is a weak acid (pH ~2.9), NH4Cl is acidic (pH ~5).

Correct: (B)

Buffers require a weak acid-base conjugate pair to neutralize added H+ or OH-; strong acids/bases cannot form effective buffers.

Correct: (B)

Using Henderson-Hasselbalch: pH = pKa + log([base]/[acid]) = 4.74 + log(1) = 4.74.

Correct: (C)

At the equivalence point only the conjugate base remains, which hydrolyzes to produce OH-, making the solution basic (pH > 7).

Correct: (B)

A negative Gibbs free energy change (ΔG < 0) indicates a spontaneous process under constant T and P.

Correct: (B)

Set ΔG = 0: T = ΔH/ΔS = (-120,000 J)/(-150 J/K) = 800 K; above 800 K, the -TΔS term dominates and ΔG > 0.

Correct: (B)

In galvanic (voltaic) cells, oxidation occurs at the anode (negative electrode) and reduction at the cathode (positive electrode).

Correct: (A)

The strongest oxidizing agent has the most positive reduction potential; F2 (+2.87 V) is the strongest oxidizer listed.

Correct: (D)

E°cell = E°cathode – E°anode = (+0.34 V) – (-0.76 V) = +1.10 V.

Correct: (B)

Alpha decay emits a helium nucleus (2 protons, 2 neutrons), decreasing Z by 2 and mass number by 4.

Correct: (C)

Nitrous oxide (N2O) is known as laughing gas due to its euphoric effects; it is also used as a dental anesthetic.