At a certain temperature, the solubility of strontium arsenate, sr3(aso4)2, is 0.0650 g/l. what is the ksp of this salt at this temperature?

Answer is: Ksp for strontium arsenate is 2.69·10⁻¹⁸.
Balanced chemical reaction (dissociation):
Sr₃(AsO₄)₂(s) → 3Sr²⁺(aq) + AsO₄³⁻(aq).
s(Sr₃(AsO₄)₂) = 0.0650 g/L.
s(Sr₃(AsO₄)₂) = 0.0650 g/L ÷ 540.7 g/mol = 1.2·10⁻⁴ mol/L.
s(Sr²⁺) = 3s(Sr₃(AsO₄)₂).
s(AsO₄³⁻) = 2s(Sr₃(AsO₄)₂).
Ksp = s(Sr²⁺)³ · s(AsO₄³⁻)².
Ksp = (3s)³ · (2s)².
Ksp = 108s⁵.
Ksp = 108 · (1.2·10⁻⁴ mol/L)⁵ = 2.69·10⁻¹⁸.