[Rutherford]

The equation 5) I was talking about is:
[H⁺]+[Ca²⁺]=[C₂O₄^2-]+[H₂C₂O₄]
In both cases (mine and the "right answer") [H₂C₂O₄] can be neglected because it is very small. I meant by solubility the concentration of [C₂O₄^2-], when I put it in the equation 5) to calculate [Ca²⁺], and then multiply [Ca²⁺] and [C₂O₄^2-], I get the value of K_sp same as it is given in the problem. When I put their answer, I get that K_sp=5.77*10^-9. So, their answer should be wrong then.
Only one issue: Is it more correct to use the concentration of Ca²⁺ or the one of C₂O₄^2- as the solubility?

[Borek]

Only one issue: Is it more correct to use the concentration of Ca²⁺ or the one of C₂O₄^2- as the solubility?

Easier to use Ca²⁺, as - technically - you should use sum of concentrations of all three forms of oxalic acid to calculate solubility.

[Rutherford]

Okay, then the solubility is 1.83*10^-3g/dm³ when using [Ca²⁺]*40. Thanks for the help.

[Rutherford]

Wait, is the mass solubility=concentration of Ca²⁺ multiplied by molar mass of Ca or molar mass of CaC₂O₄?

[Borek]

Mass solubility of what?

[Rutherford]

It's the mass solubility of calcium oxalate monohydrate  . When I multiply the concentration of [Ca²⁺] with the molar mass of calcium oxalate monohydrate I get the answer they gave. So, there I mistaken  .

[Rutherford]

So, I got:
1)K_sp=[Ca²⁺][C₂O₄^2-]
2)K_a1K_a2=[C₂O₄^2-][H⁺]²/[H₂C₂O₄]
3)[H⁺]+2[Ca²⁺]=[HC₂O₄^-]+2[C₂O₄^2-]
4)[Ca²⁺]=[C₂O₄^2-]+[H₂C₂O₄]+[HC₂O₄^-]
I will try to solve this now.

Why in the charge equality equation (3) I didn't use the concentration of OH^- ions, but still got the right answer? Should it be used?

[Borek]

It should be there, but as it is orders of magnitude lower than the concentrations of other ions, the error didn't matter much.

[Rutherford]

It becomes really hard to solve this when taking into account the concentration of the hydroxy ions, but I think that it should be that way because I can't see at the beginning whether its concentration matters (its not too much lower than the one of hydrogen ions). Probably, at some point, it can be neglected. Thanks.