Calculate Solubility of Ions given Ksp

Question

The Ksp of Fe₃(PO₄)₂ is 2.50⋅10^−20

M.

Calculate the solubility of each ion in a saturated solution: (Fe^2+ and PO_4^3-)

I'm confused on this question and don't know where to start. Any help is appreciated.

Answer 1

Always start by writing out the equation:

Fe₃(PO₄)₂(s) <==> 3Fe²⁺(aq) + 2PO₄^3-(aq)

Then, write the Ksp expression:

Ksp = [Fe²⁺]³[PO₄^3-]²

Next, assign variables. In this case we can let [PO₄^3-] = x and then [Fe²⁺] = 1.5x

Now substitute the values into the Ksp equation:

2.50x10^-20 = (1.5x)³(x)²

2.5x10^-20 = 3.38x⁵

x⁵ = 7.4x10^-21

x = 9.4x10^-5 M = [PO₄^3-]

[Fe²⁺] = 1.5x = 1.5 * 9.4x10^-5 M = 1.4x10^-4 M