A solution in equilibrium with solid barium phosphate is found to have a barium ion concentration of?

Question

5 x 10^ -4 mol/L and a Ksp of 5 x 10^-23. Calculate the concentration of phosphate ion.

Answer 1

Ba3(PO4)2 -----  3 Ba+2 + 2 PO4^-3

since a Ksp is equal to the concentrations of the products raised to the power of their coefficients  then

Ksp = [Ba+2]^3 [PO4]^2

so   (5 X 10^-4 )^3( PO4)^2 = 5 X 10^-23

(PO4)^2 = 5 X 10^-23 / 125 X 10 ^-12

PO4 ^-3 = 6.32 X 10^-7 M