Question

Question #b0189

Answer 1

The mass-balance equation is `2["Sr"^"2+"] = 3["PO"_4^"3-"]`.

Explanation:

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 `s` of strontium phosphate.

You would probably start by writing something like this:

`color(white)(mmmmmm)"Sr"_3("PO"_4)_2"(s)" ⇌ "3Sr"^"2+""(aq)" + "2PO"_4^"3-""(aq)"`
`"E/mol·L"^"-1":color(white)(mmmmmmmmmmll)3scolor(white)(mmmmml)2s`

You are saying that

`["Sr"^"2+"] = 3s` and `["PO"_4^"3-"] = 2s`

Then,

`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.