The solubility of Fe2S3 is measured and found to be 2.16×10-16 g/L. Use this information to calculate a Ksp value for iron(III) sulfide.

Question

The solubility of Fe₂S₃ is measured and found to be 2.16×10^-16 g/L. Use this information to calculate a K_sp value for iron(III) sulfide.

K_sp = 

Answer 1

To calculate the Ksp, we'll want to convert g/L to moles/L (M).

2.16x10^-16 g/L x 1 mol Fe₂S₃/208 g = 1.04x10^-18 mole/L

Fe₂S₃(s) ==> 2F³⁺(aq) + 3S^2-(aq)

Ksp = [Fe³⁺]²[S^2-]³

[Fe³⁺] = 2 x 1.04x10^-18 = 2.08x10^-18

[S^2-] = 3 x 1.04x10^-18 = 3.12x10^-18

Ksp = (2.08x10^-18)²(3.12x10^-18)³

Ksp = 1.31x10^-88 (that's one insoluble compound)