[Anthasci]

But doesnt the concentration of all forms of the oxalic acid equal the concentration of Zn²⁺ ions? Which in turn, equals the Ksp divided by the concentration of A^2-?

That is assuming Ksp=[Zn²⁺] [A^2-]

[Borek]

Good point, you were right. I misunderstood your understanding

[Anthasci]

Ok then  

Using the 9.13 equation with K_a1 = 0,0556 and K_a2 = 5,56*10^-5 I get a result of 0,0187 g/L

What I did - after calculating the [A^2-] from 9.13, I derived the [Zn²⁺] = s. I then multiplied this by the molar mass of ZnC₂O₄ and got the result. The solution however, was supposed to be 0,0576 g/L. Are they wrong or did I mix something up?

EDIT: Oh yeah, just noticed about the stepwise thingie ... Got the correct result now. Guess I'll have to work on these derivations a bit   I think I understand these system a little better now. I'll always try to write the balance of masses and the balance of charges and go from there (final exam of analitical chemistry on wednesday, it'll be tough, considering we also have the instrumental analysis ...)

One more question though - equation 9.5 on your link - I always count both [H⁺] and [OH^-] ions, since they're both always present in a solution, right? Same would go if it were basic, then I'd just write [H₃O⁺] in the terms of Kw and [OH^-]