The mass-balance equation is
In a solubility product calculation, a mass-balance equation relates the equilibrium concentrations of the different species in the solution to each another.
For example, assume that you were asked to calculate the molar solubility
You would probably start by writing something like this:
You are saying that
#color(blue)(2["Sr"^"2+"] = 3["PO"_4^"3-"])#
This is the mass-balance equation.
In effect, to get the mass-balance equation, you multiply each concentration by the charge on the ion.