AP Chemistry Questions: Calculating pH of a Buffer After Adding Acid

📚 Quick Study Guide

🔍 Buffers resist changes in pH when small amounts of acid or base are added. 🧪 Buffers are typically made from a weak acid (HA) and its conjugate base (A⁻), or a weak base and its conjugate acid. 📝 The Henderson-Hasselbalch equation is key: $pH = pK_a + log(\frac{[A^-]}{[HA]})$ ➗ When adding a strong acid ($H^+$), it reacts with the conjugate base ($A^−$) to form the weak acid ($HA$). ⚖️ Write out the reaction: $H^+ + A^- \rightarrow HA$. ➕ After the reaction, calculate the new concentrations of $A^−$ and $HA$. It's an ICE table situation! 🔢 Plug the new concentrations into the Henderson-Hasselbalch equation to find the new pH.

🧪 Practice Quiz

1. A buffer solution contains 0.30 M $CH_3COOH$ and 0.20 M $CH_3COO^-$. If 0.05 moles of HCl is added to 1.0 L of this buffer, what is the resulting pH? ($K_a$ of $CH_3COOH = 1.8 \times 10^{-5}$) A. 4.57 B. 4.92 C. 4.63 D. 4.32 2. A buffer is prepared by mixing 25.0 mL of 0.100 M benzoic acid ($C_6H_5COOH$, $K_a = 6.3 \times 10^{-5}$) with 75.0 mL of 0.200 M sodium benzoate ($C_6H_5COONa$). What is the pH after adding 5.0 mL of 1.00 M HCl? A. 3.89 B. 4.02 C. 4.35 D. 4.51 3. Which of the following statements is correct regarding the effect of adding a strong acid to a buffer solution? A. The pH will always significantly decrease, regardless of the buffer capacity. B. The concentration of the weak acid component will decrease. C. The concentration of the conjugate base component will increase. D. The ratio of conjugate base to weak acid decreases. 4. A buffer solution contains 0.25 M $NH_3$ and 0.45 M $NH_4Cl$. What is the pH after the addition of 0.020 mol of HCl to 500.0 mL of this buffer? ($K_b$ for $NH_3 = 1.8 \times 10^{-5}$) A. 9.15 B. 9.32 C. 8.98 D. 9.47 5. A solution is prepared by mixing 50.0 mL of 0.20 M $HCN$ and 50.0 mL of 0.10 M $NaCN$. What is the pH of the solution after adding 10.0 mL of 1.0 M $HCl$? ($K_a$ for $HCN = 4.9 \times 10^{-10}$) A. 9.31 B. 9.15 C. 8.95 D. 9.45 6. What is the change in pH when 0.010 mol of HCl is added to 1.0 L of a buffer solution that is 0.20 M in $CH_3COOH$ and 0.20 M in $CH_3COONa$? ($K_a = 1.8 \times 10^{-5}$) A. -0.046 B. -0.092 C. -0.023 D. -0.010 7. A buffer solution contains equal concentrations of a weak acid and its conjugate base. Adding a strong acid will cause the pH to: A. Increase significantly. B. Decrease significantly. C. Change slightly. D. Remain constant.

Click to see Answers

1. C
2. A
3. D
4. B
5. A
6. A
7. C