I didn't finished my reply my internet was cut.

Now...

That typo you gave to me the Hederson one exists only when the approximations work.

Here after ionizations and with C H_{2}SO_{4}'s concentration we have:

1) C+x=10^{-4} (from the ph)

2) 10^{-2}=x*10^{-4}/C-x (from the second ionization)

Well that Hederson typo is the 2) but with the approximations:

3) 10^{-2}=x*10^{-4}/C

If we solve 1-3 we find that C=x which is impossible

If we solve the 1-2 we take "reasonable" results which I'll write down tomorrow...

I hope you understand... Goodnight...