solubility question?

Question

The solubility of calcium arsenate (Ca3(AsO4)2, molar mass = 398.078 g) in water is measured to be 0.034 g/L. What is Ksp for this salt?

Answer 1

If s is the solubility of Ca3(AsO4)2 in mole/L then

... ... .. .. Ca3(AsO4)2 <=> 3Ca(+2) + 2AsO4(-3)
Dissolve .. ... s
Produce .. .. .. .. .. . .. . . ... .. .. .3s .. .. .. . 2s

Ksp=[Ca+2]^3 [AsO4-3]^2= (3s)^3 *(2s)^2= 27s^3 * 4s^2 =>

Ksp =108s^5

The solubility of Ca3(AsO4)2 is
0.034/398.078 =8.54*10^-5 M

Thus Ksp =108*(8.54*10^-5)^5 =4.9*10^-19