I didn't finished my reply my internet was cut.
That typo you gave to me the Hederson one exists only when the approximations work.
Here after ionizations and with C H2SO4'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:
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...