[gidget]

Write the mass-balance expression and the Charge-Balance equations for a solution that is:
a) 0.10M in Na₂HPO₄
b) 0.25M in NaF and saturated with CaF₂
c) Saturated with MgCO₃

* I am totally lost on these since the prof hadn't gone over any examples in the lecture. Any help would be GREATLY appreciated! Thank you!

[Borek]

here goes an example:

You have a 0.1M solution of phosphoric acid.

Mass balance (sum of all forms of acid present equals its initial concentration):

[H₃PO₄] + [H₂PO₄^-] + [HPO₄^2-] + [PO₄^3-] = 0.1

Charge balance (solution is neutral):

[H₂PO₄^-] + 2[HPO₄^2-] + 3[PO₄^3-] + [OH^-] = [H⁺]

If you have solids present you may need to express mass balance in terms of not concentrations, but numbers of moles.