AP Chemistry Questions on Buffer Solutions

📚 Quick Study Guide

• ⚖️ A buffer solution resists changes in pH upon addition of small amounts of acid or base.
• 🧪 Buffers are typically made from a weak acid and its conjugate base, or a weak base and its conjugate acid.
• ⚗️ The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: $pH = pK_a + log(\frac{[A^-]}{[HA]})$ where $A^-$ is the conjugate base and $HA$ is the weak acid.
• ➕ When a strong acid is added to a buffer, it reacts with the conjugate base, neutralizing it.
• ➖ When a strong base is added to a buffer, it reacts with the weak acid, neutralizing it.
• 🔢 The buffer capacity is the amount of acid or base a buffer can neutralize before significant pH change occurs.
• 🌡️ Temperature affects buffer solutions, as it changes the $K_a$ and $K_b$ values.

Practice Quiz

1. Which of the following mixtures would create a buffer solution when dissolved in 1.0 L of water?
   1. A. 0.20 mol $HCl$ and 0.20 mol $NaCl$
   2. B. 0.20 mol $NaOH$ and 0.20 mol $NaCl$
   3. C. 0.20 mol $CH_3COOH$ and 0.20 mol $CH_3COONa$
   4. D. 0.20 mol $HCl$ and 0.40 mol $NaOH$
2. What is the pH of a buffer solution that is 0.30 M in acetic acid ($CH_3COOH$) and 0.30 M in sodium acetate ($CH_3COONa$)? The $K_a$ for acetic acid is $1.8 \times 10^{-5}$.
   1. A. 4.74
   2. B. 9.26
   3. C. 7.00
   4. D. 3.52
3. Which of the following statements about buffer solutions is FALSE?
   1. A. A buffer solution resists changes in pH when small amounts of acid or base are added.
   2. B. A buffer solution contains a weak acid and its conjugate base.
   3. C. The pH of a buffer solution is always 7.
   4. D. The buffer capacity can be exceeded if too much acid or base is added.
4. A buffer solution is prepared by mixing 25.0 mL of 0.100 M $NH_3$ with 25.0 mL of 0.200 M $NH_4Cl$. What is the pH of the resulting solution? ($K_b$ for $NH_3 = 1.8 \times 10^{-5}$)
   1. A. 9.25
   2. B. 4.75
   3. C. 9.55
   4. D. 4.45
5. What happens to the pH of a buffer solution when it is diluted with water?
   1. A. The pH increases.
   2. B. The pH decreases.
   3. C. The pH remains the same.
   4. D. It depends on the buffer concentration.
6. Which of the following acids would be most suitable for preparing a buffer with a pH of 3.5?
   1. A. Acetic acid ($K_a = 1.8 \times 10^{-5}$)
   2. B. Formic acid ($K_a = 1.8 \times 10^{-4}$)
   3. C. Hypochlorous acid ($K_a = 3.0 \times 10^{-8}$)
   4. D. Hydrofluoric acid ($K_a = 6.8 \times 10^{-4}$)
7. What is the effect of adding a strong acid to a buffer solution containing a weak acid ($HA$) and its conjugate base ($A^-$)?
   1. A. The concentration of $HA$ increases and the concentration of $A^-$ decreases.
   2. B. The concentration of $HA$ decreases and the concentration of $A^-$ increases.
   3. C. Both the concentrations of $HA$ and $A^-$ increase.
   4. D. Both the concentrations of $HA$ and $A^-$ decrease.