Using the Henderson-Hasselbalch equation, what is the pH of a buffer solution that is 0.250 M in HCN and 0.170 M in KCN, given that the pKa of HCN is 9.31?

• A. 7.00
• B. 8.99
• C. 9.31
• D. 9.63

Answer: (B)

Verified step by step guidance

1. Identify the components of the buffer solution: HCN is the weak acid and KCN provides the conjugate base, CN⁻.
2. Recall the Henderson-Hasselbalch equation: ${\mathrm{pH}}_{}={\mathrm{pK}}_a+\log \left(\frac{\mathrm{[A⁻]}}{\mathrm{[HA]}}\right)$, where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.
3. Substitute the given values into the equation: ${\mathrm{pH}}_{}=9.31+\log \left(\frac{0.170}{0.250}\right)$.
4. Calculate the ratio of the concentrations: $\frac{0.170}{0.250}$.
5. Use the logarithm of the ratio to find the pH: Add the result to the pKa value to determine the pH of the buffer solution.