Chem 2 Problem Acid/Base

Question

For the diprotic acid, H2A, Ka1 = 3.6 x 10-5 and Ka2 = 2.6 x 10-12 at a certain temperature. What is the pH of a 0.124 M solution of the acid at this temperature?

Answer 1

H2A <-----> H+ + A-

3.6 x 10^-5 = x^2/ 0.124 -x

x = [H+] =0.00211 M

pH = 2.68

The 2nd dissociation does not affect [H+] because Ka2 is  small