[UG]

The question I am stuck on simply goes: The dissociation constants of acid A are 10^–3.46 an 10^–5.10. Calculate the pH of a 0.100 M solution of A.
So I've got [H₂A] + [HA^-] + [A^2-] = 0.100
[HA^-] + 2[A^2-] = [H⁺] I am ignoring [OH-] here because the solution is going to be around 2-3 pH and the two K_a expressions. After a couple of pages of working, I ended up with

[HA^-] = 0.10 x K_a1[H⁺] / ([H⁺]² + K_a1[H⁺] + K_a2)

and [A^2-] = K_a2 x 0.10 / ([H⁺]² + K_a1[H⁺] + K_a2)

Substituting these into the charge balance gives
[0.10 x K_a1[H⁺] / ([H⁺]² + K_a1[H⁺] + K_a2)] + [2(K_a2 x 0.10 / ([H⁺]² + K_a1[H⁺] + K_a2))] = [H⁺]

I don't want to get to far ahead of myself, is my working correct up to this stage?

[Borek]

Most likely OK.

[UG]

Thanks, but I see where I went wrong.

[Borek]

So you are better than me.