AP Chemistry Questions on Weak Acids and Bases: Exam Prep

📚 Quick Study Guide

• ⚖️ Weak acids and bases only partially dissociate in water.
• 📝 The acid dissociation constant, $K_a$, measures the strength of a weak acid: $HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)$, where $K_a = \frac{[H_3O^+][A^-]}{[HA]}$.
• 🧪 Similarly, the base dissociation constant, $K_b$, measures the strength of a weak base: $B(aq) + H_2O(l) \rightleftharpoons BH^+(aq) + OH^-(aq)$, where $K_b = \frac{[BH^+][OH^-]}{[B]}$.
• ➗ The relationship between $K_a$ and $K_b$ for a conjugate acid-base pair is: $K_a \cdot K_b = K_w = 1.0 \times 10^{-14}$ at 25°C.
• ➕ pH can be calculated from $[H_3O^+]$ using the formula: $pH = -log[H_3O^+]$.
• ➖ pOH can be calculated from $[OH^-]$ using the formula: $pOH = -log[OH^-]$.
• 🌡️ $pH + pOH = 14$ at 25°C.
• 💡 The Henderson-Hasselbalch equation is useful for calculating the pH of a buffer solution: $pH = pK_a + log(\frac{[A^-]}{[HA]})$.

🧪 Practice Quiz

1. Question 1: Which of the following is a weak acid?
   1. A) HCl
   2. B) $H_2SO_4$
   3. C) $HNO_3$
   4. D) $CH_3COOH$
2. Question 2: The $K_a$ of a weak acid HA is $2.0 \times 10^{-5}$. What is the pH of a 0.1 M solution of HA?
   1. A) 2.35
   2. B) 2.87
   3. C) 3.22
   4. D) 3.70
3. Question 3: What is the conjugate base of $H_2PO_4^-$?
   1. A) $H_3PO_4$
   2. B) $HPO_4^{2-}$
   3. C) $H^+$
   4. D) $PO_4^{3-}$
4. Question 4: A solution of a weak base has a pH of 11.0. What is the pOH of this solution?
   1. A) 1.0
   2. B) 2.0
   3. C) 3.0
   4. D) 4.0
5. Question 5: Which of the following buffer solutions would be most effective at pH 5.0? (The $pK_a$ values are given in parentheses.)
   1. A) Acetic acid/acetate (4.76)
   2. B) Formic acid/formate (3.75)
   3. C) Hypochlorous acid/hypochlorite (7.53)
   4. D) Ammonium/ammonia (9.25)
6. Question 6: What is the hydroxide ion concentration, $[OH^-]$, in a solution with a pOH of 4.5?
   1. A) $3.16 \times 10^{-5}$ M
   2. B) $3.16 \times 10^{-10}$ M
   3. C) $3.16 \times 10^{-9}$ M
   4. D) $3.16 \times 10^{-4}$ M
7. Question 7: The $K_b$ for ammonia ($NH_3$) is $1.8 \times 10^{-5}$. What is the $K_a$ for its conjugate acid, ammonium ($NH_4^+$)?
   1. A) $1.8 \times 10^{-5}$
   2. B) $5.6 \times 10^{-10}$
   3. C) $1.0 \times 10^{-14}$
   4. D) $1.0 \times 10^{-7}$