Question

Calculate the [Ca⁺²] in solution over solid Ca₃(PO₄)₂ . The pK_s =28.7.

Answer 1

i think the solubility product of Ca₃(PO₄)₂ should be 2.8x10^-7 (you have mistaken!)

Ca₃(PO₄)₂ =  3Ca⁺ + 2PO₄^3-

Ksp = [Ca²⁺]³[PO4^3-]²

2.8x10^-7 = [3x]³[2x]²

x = 0.019

so concentration of  Ca⁺²  i.e.  [Ca⁺²] = 3x = 0.057 mol/L