What is the pH of a H_{2}C_{2}O_{4} solution (c=0.2M) at the 1st and the 2nd equivalence point if it was titrated with 0.2M NaOH? K_{a1}=5.4*10^{-2}, K_{a2}=5.4*10^{-5}.

At the 1st point I have to calculate the pH of a HC_{2}O_{4}^{-} solution. pH=(pK_{a1}+pK_{a2})/2=2.78, but how to calculate the pH for the second point? After the first step the volume of the solution will increase twice so c(HCO_{4}^{-})=0.1M, here I stopped.