[DeltaSubz]

Hey all, first post here 
Anyways, it seems there will be some mass/charge balance questions on my upcoming quant midterm and want to make sure i get this right. Im pretty sure i got the hang of mass balance, but not so much charge balance. An example problem is as follows, and my answers are in red...

A solution is prepared using 0.01 mol of Na₂HPO₄, 0.02 mol of Ca(H₂PO4)₂, and 0.02 mol of Mg₃(PO₄)2. In a total volume of  0.50 L.

a.   Write all mass balance equations for this solution
[Na+] = 0.04 M
[Ca²⁺] = 0.04 M
[Mg²⁺] = (3*0.02)/0.5 = 0.12 M
[H₃PO₄] + [H₂PO₄^-] + [HPO₄^2-] + [PO₄^3-] = ( 0.01 + (2*0.02) + (2*0.02))/0.5 = 0.18 M

b.   Write the charge balance equation for this solution. Where appropriate, replace general concentration terms with actual concentrations.
[H₃O⁺] + (0.36 M) = [H₂PO₄^-] + 2[HPO₄^2-] +3[PO₄^3-] + [OH^-]

Can anyone knowledgeable in this area verify whether this is right/wrong?

[Borek]

I just skimmed over your numbers and nothing sticks out as obviously incorrect, but there is one thing that I would do differently:

[H₃O⁺] + (0.36 M) = [H₂PO₄^-] + 2[HPO₄^2-] +3[PO₄^3-] + [OH^-]

Instead of writing 0.036 M I would first list all ions with coefficients. Makes it easier to check what you are doing.

[AWK]

Charge balance and mass balance will be correct assuming that all compounds are soluble in water and do not react chemically between each other.
But this is not  the case. The solubility of Mg₃(PO₄)₂ is of the order of 10^-6 M.