Calculate the equilibrium concentration of hc2o4− in a 0.20 m solution of oxalic acid.

For the first dissociation of H2C2O4 ↔ H+ + HC2O4-, pKa = 1.23
Ka = 10^-1.23
Ka = 0.0589

Ka = [H+] [ HC2O4-] / [H2C2O4]
Call [H+] = X which will also = [HC2O4-] and that [H2C2O4] = (0.2-X )
So we can write:
0.0589 = X² / (0.2-X)
X² = 0.0589( 0.2 - X)
X² = 0.01178 - 0.0589X
X² + 0.0589X - 0.01178 = 0
Solve quadratic for X: Positive root = 0.083

The molar concentration of HC2O4- at equilibrium = 0.083M

This is from yahoo. Credit to Trevor H