Question 10.13: What is the pH of a buffer solution that contains 0.100 M HF...
What is the pH of a buffer solution that contains 0.100 M HF and 0.120 M NaF? The K_a of HF is 3.5 \times 10^{-4}, and so pK_a = 3.46.
ANALYSIS The Henderson Hasselbalch equation can be used to calculate the pH of a buffer solution: pH = pK_a + \log \left(\frac{[F^-]}{[HF]} \right) .
BALLPARK ESTIMATE If the concentrations of F^- and HF were equal, the log term in our equation would be zero, and the pH of the solution would be equal to the pK_a for HF, which means pH = 3.46. However, since the concentration of ([F^-] = 0.120 M) the conjugate base is slightly higher than the concentration of the conjugate acid ([HF] = 0.100 M) , then the pH of the buffer solution will be slightly higher (more basic) than the pK_a.
BALLPARK CHECK The calculated pH of 3.54 is consistent with the prediction that the final pH will be slightly higher than the pK_a of 3.46.