Calculate the pH of a solution that is 0.205M benzoic acid and 0.230M sodium benzoate, a salt whose anion is the conjugate base of benzoic acid. (The pKa of benzoic acid

pH = 4.25

Explanation:

A solution composed of a weak acid and its conjugate base is a buffer solution. To calculate the pH of a buffer solution we use the Henderson-Hasselbach equation:

pH = pKa + log ([conjugate base]/[weak acid]

In this case, we have the following data:

[conjugate base] = [sodium benzoate] = 0.230 M

[weak acid] = [benzoic acid] = 0.205 M

The pKa of benzoic acid is 4.2. So, we introduce the data in the equation:

pH = 4.2 + log (0.230 M/0.205 M) = 4.2 + 0.050 = 4.25