A buffer contains the weak acid HA and its conjugate base A⁻. The weak acid has a pKa of 5.25 and the buffer has a pH of 4.82. Which statement is true of the relative concentrations of the weak acid and the conjugate base in the buffer?

• A. The concentrations of HA and A⁻ are unrelated to the pH and pKa.
• B. The concentration of HA is less than the concentration of A⁻.
• C. The concentration of HA is equal to the concentration of A⁻.
• D. The concentration of HA is greater than the concentration of A⁻.

Answer: (D)

Verified step by step guidance

1. To determine the relative concentrations of the weak acid (HA) and its conjugate base (A⁻) in the buffer, we can use the Henderson-Hasselbalch equation, which is: ${\mathrm{pH}}_{}={\mathrm{pK}}_a+\log \left(\frac{A_{-}}{{\mathrm{HA}}_{}}\right)$
2. Given that the pH of the buffer is 4.82 and the pKa of the weak acid is 5.25, we can substitute these values into the Henderson-Hasselbalch equation: $4.82=5.25+\log \left(\frac{A_{-}}{{\mathrm{HA}}_{}}\right)$
3. Rearrange the equation to solve for the ratio of the concentrations of A⁻ to HA: $\log \left(\frac{A_{-}}{{\mathrm{HA}}_{}}\right)=4.82-5.25$
4. Calculate the difference between the pH and pKa: $4.82-5.25=-0.43$. This negative value indicates that the concentration of HA is greater than the concentration of A⁻.
5. Since the logarithm of the ratio is negative, it implies that the ratio $\frac{A_{-}}{{\mathrm{HA}}_{}}$ is less than 1, meaning the concentration of HA is greater than the concentration of A⁻.