[neon]

What are the concentrations of H⁺, H₂C₂O₄, HC₂O₄^- and C₂O₄^2- in a 0.1 M solution of oxalic acid. K1=10^-2 M and K2 = 10^-5

H₂C₂O₄    H⁺ + HC₂O₄^-
0.1-x           x+y         x

HC₂O₄^-  H⁺ +  C₂O₄^2-
x-y             x+y        y

[itex]K_1 = \dfrac{x(x+y)}{0.1-x} [/itex]

Neglecting x from denominator and y from numerator, I get x=0.031 which is incorrect. Can anyone help? I'm really dumb at ionic equilibrium. 

[Borek]

Ka2 is three orders of magnitude lower than Ka1, you can safely assume second dissociation doesn't matter for pH.