Weak Diprotic Acid: maleic acid

[Aackson]

Maleic acid is a weak diprotic acid with a pKa1 = 1.87 and pKa2 = 6.07.


10mL 0.1M maleic acid is titrated with 0.1M NaOH.

Calculate the volume of NaOH required to reach each equivalence point, and calculate the pH at each equivalence point. 


ok here's as far as I've got:

I've converted the pKa values to Ka values using Ka=10^-pKa
thus, Ka1 = 1.35E-2
        Ka2 = 8.51E-7

maleic acid which I will represent as H2A ionizes in a 2 step process.

H2A <-> H+ +HA-
HA- <-> H+ + A2-

then I did this:

Ka1 = 1.35E-2 = [H+][HA-] / [H2A]
Ka2 = 8.51E-7 = [H+][A2-] / [HA-]

and thats as far as I've got at the moment, a push in the right direction would be appreciated!