Question

Calculate the pH of a solution formed by mixing 150.0 mL of 0.10 M HC7H5O2 with 100.0 mL of 0.30 M NaC7H5O2. The Ka for HC7H5O2 is 6.5 × 10-5.

Answer 1

This is a buffer solution that you're creating.

Before the NaC7H5O2 is added, the following equilibrium was already been established:\[HC_7H_5O_2(aq)+H_2O(l) \rightarrow H_3O^+(aq)+C_7H_5O_2^-(aq)\]When we add the conjugate base to the solution, we are shifting the equilibrium to towards the left. To find the final concentration of hydronium (and pH), we need to set up an ICE chart:

We can assume that the dissociation of the acid is negligible, as well as the association of the base, and plug these numbers into the Henderson-Hasselbalch equation:\[pH=pKa+\log(\frac{[A^-]}{[HA]})=4.19+\log(\frac{0.12}{0.06})=4.49\]