What is the pH of a buffer solution that is 0.185 M in lactic acid and 0.132 M in sodium lactate? The Ka of lactic acid is 1.4 × 10^-4.

• A. 3.85
• B. 5.10
• C. 2.95
• D. 4.25

Answer: (A)

Verified step by step guidance

1. Identify the components of the buffer solution: lactic acid (CH₃CH(OH)COOH) and its conjugate base, sodium lactate (CH₃CH(OH)COONa).
2. Use the Henderson-Hasselbalch equation to calculate the pH of the buffer solution: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{A}^-]}{[\text{HA}]} \right) \), where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the acid.
3. Calculate the pKa from the given Ka of lactic acid: \( \text{pKa} = -\log(\text{Ka}) \). Substitute \( \text{Ka} = 1.4 \times 10^{-4} \) into the equation to find pKa.
4. Substitute the concentrations of sodium lactate (0.132 M) and lactic acid (0.185 M) into the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{0.132}{0.185} \right) \).
5. Solve the equation to find the pH of the buffer solution.