Question

What is the pH of a 0.0350 M solution of benzoic acid?

Answer 1

Benzoic acid is a weak acid and it will dissociate partially in water to give H⁺ ions. Reaction can be shown as follows:

HA = H⁺ + A^-

Let s be the amount of HA dissociated. Thus, concentration of remaining HA becomes (0.0350 – s) and that of H⁺ and A^- is s

Thus, expression for Ka can be written as follows:

Ka = [H^+][A^-] / [HA]

Ka for benzoic acid is 6.4 x 10^-5

6.4 x 10^-5 = s x s / (0.0350 – s)

Since, benzoic acid is weak acid, s can be assumed to be very small, therefore,

6.4 x 10^-5 = s x s / 0.0350

s2 = 6.4 x 10^-5 x 0.0350

s2 = 0.224 x 10^-5

s = 0.001497 M

Therefore, [H⁺] = 0.001497 M

Expression for pH can be written as follows:

pH = -log[H+]

pH = -log (0.001497)

pH = 2.82

Thus, pH of 0.0350 M solution of benzoic acid is 2.82