Chapter 3 — Quiz
Twenty questions. Answers at the end.
1. A Brønsted acid: (a) accepts an electron pair. (b) donates a proton. (c) accepts a proton. (d) donates an electron pair.
2. The relationship between $pK_a$ and $K_a$ is: (a) $pK_a = K_a$ (b) $pK_a = \log K_a$ (c) $pK_a = -\log K_a$ (d) $pK_a = -K_a$
3. A small $pK_a$ indicates: (a) a weak acid. (b) a strong acid. (c) a weak base. (d) a strong base.
4. Which is the strongest acid? (a) $HF$ ($pK_a = 3.2$) (b) acetic acid ($pK_a = 4.76$) (c) phenol ($pK_a = 10$) (d) ethanol ($pK_a = 16$)
5. The approximate $pK_a$ of a carboxylic acid is: (a) $-7$ (b) $5$ (c) $10$ (d) $15$
6. The approximate $pK_a$ of an alcohol is: (a) $0$ (b) $5$ (c) $10$ (d) $16$
7. The approximate $pK_a$ of a terminal alkyne C–H is: (a) $5$ (b) $16$ (c) $25$ (d) $50$
8. Acetate ($CH_3CO_2^-$) is more stable than ethoxide ($CH_3CH_2O^-$) primarily because: (a) Acetate has more electronegative atoms overall. (b) The negative charge in acetate is delocalized over two oxygens by resonance. (c) Acetate contains a carbonyl group. (d) Acetate is smaller.
9. Trichloroacetic acid ($pK_a = 0.65$) is much more acidic than acetic acid ($pK_a = 4.76$) because: (a) Resonance. (b) The inductive effect of the three chlorines stabilizes the carboxylate. (c) The chlorines change the hybridization. (d) Solvation is better in chlorinated solvents.
10. A terminal alkyne C–H is about $10^{25}$ more acidic than an alkane C–H because: (a) The triple bond is shorter. (b) The sp hybrid holds electrons closer to the nucleus. (c) Alkynes are unsaturated. (d) Alkane C-H bonds are stronger.
11. In an acid-base reaction, the equilibrium favors: (a) The side with the stronger acid. (b) The side with the weaker acid. (c) Whichever side has more atoms. (d) Whichever side is neutral.
12. For the reaction $HA + B^- \rightleftharpoons A^- + HB$, if $pK_a(HA) = 5$ and $pK_a(HB) = 15$, then $K_{eq}$ is: (a) $10^{-20}$ (b) $10^{-10}$ (c) $10^{10}$ (d) $10^{20}$
13. Which base cannot effectively deprotonate a terminal alkyne? (a) $NaNH_2$ (sodium amide) (b) $BuLi$ (n-butyllithium) (c) $NaOH$ (sodium hydroxide) (d) LDA
14. A Lewis acid is: (a) A proton donor. (b) A proton acceptor. (c) An electron-pair donor. (d) An electron-pair acceptor.
15. Which is NOT a Lewis acid? (a) $BF_3$ (b) $AlCl_3$ (c) $Fe^{3+}$ (d) $NH_3$
16. The best leaving group among the following is: (a) $F^-$ (b) $Cl^-$ (c) $HO^-$ (d) $I^-$
17. Leaving-group ability correlates with: (a) The $pK_a$ of the conjugate acid — lower $pK_a$ means better LG. (b) The $pK_a$ of the conjugate acid — higher $pK_a$ means better LG. (c) The size of the leaving group only. (d) The color of the leaving group.
18. Which is the most common $\alpha$-hydrogen $pK_a$ in a simple ketone? (a) ~5 (b) ~10 (c) ~20 (d) ~50
19. A protonated amine ($R_3NH^+$) has $pK_a \approx$: (a) $-2$ (b) $5$ (c) $10$ (d) $16$
20. In physiological conditions (pH 7.4), a carboxylic acid with $pK_a = 4$ is: (a) Mostly protonated (neutral). (b) Mostly deprotonated (negatively charged). (c) Exactly 50:50. (d) Crystalline.
Answers
- b. 2. c. 3. b. 4. a. 5. b. 6. d. 7. c. 8. b. 9. b. 10. b. 11. b. 12. c. 13. c. 14. d. 15. d. 16. d. 17. a. 18. c. 19. c. 20. b.
Scoring: 17+ = ready for Chapter 4. 13–16 = review the ARIO section and equilibrium-direction rules. Below 13 = this is the most foundational chapter; re-read before proceeding.