Solubulity Problem?

Question

Sr(NO3)2 (aq) is added to 1.0 L of solution containing 0.020 mol F- and 0.10 mol SO4(2-) at constant volume. Which salt precipitates first? What is [Sr2+] in solution when the precipitate start to form?

Ksp for SrSO4 = 7.6 x 10^-7
Ksp for SrF2 = 7.9 x 10^-10

Answer 1

For the first case: [Sr+2] = Ksp/(F-)^2 = 1.98 x 10^-6

For the second case: [Sr+2] = Ksp/(SO4-2) = 7.6 x 10^-6

SrSO4 will precipitate first at a strontium ion concentration of
7.6 x 10^-6 molar. This is about 4 x more concentrated than when SrF2 will begin to precipitate.