Weak Acids and Bases

Chloroacetic acid, ClCH2COOH, is a stronger monoprotic acid than acetic acid. In a 0.10 M solution, the pH is 1.96.
a) What is the Ka for chloroacetic acid??
b) What is the pKa for chloroacetic acid?


a)
ClCH2COOH(aq) ≒ ClCH2COO-(aq) + H+(aq)

At the beginning:
[ClCH2COOH]o = 0.10 M
[ClCH2COO-]o = [H+]o = 0 M

At equilibrium:
[H+] = 10-pH = 10-1.96 M
[ClCH2COO-] = [H+] = 10-1.96 M
[ClCH2COOH] = (0.1 - 10-1.96) M

Ka
= [ClCH2COO-][H+]/[ClCH2COOH]
= (10-1.96)2/(0.1 - 10-1.96)
= 1.35 x 10-3 M

b)
pKa
= -log(1.35 x 10-3)
= 2.87