A 0.20 M sodium chlorobenzoate (NaC7H4ClO2) solution has a pH of 8.65. Calculate the pH of a 0.19 M chlorobenzoic acid (HC7H4ClO2) solution.

Answer 1

If we call the compound HBz, then the salt is NaBz. The first part of the problem is a hydrolysis problem. The Bz part hydrolyzes according to the following equation.
Bz^- + HOH ==> HBz + OH^-

Kb = (Kw/Ka) = (HBz)(OH^-)/(HBz)
I would use the 8.65 pH, convert to pOH, and convert that to OH^- which will be equal to HBz. Substitute into the hydrolysis equation above and solve for the only unknown, Ka.

Then HBz, the acid. ==> H^+ + Bz^-

Ka = (H^+)(Bz^-)/(HBz)
Set up an ICE chart for the acid, and solve for (H^+), then convert to pH.
Post your work if you get stuck.