i feel like none of these answers are right.

if the solubility of the solid is 'S' mol/ L, what is the Ksp of the product of the salt?

Ca3PO42-(s) --> 3Ca2+(aq) + 2PO42- (aq)

A)ksp=S^2
B)ksp=4S^3
C)ksp=27S^4
D)ksp=108S^5

Only leaning towards D because its the only one that makes sense...

ksp=[3Ca2+]^3[2PO42-]^2 ????
ksp=108S^5

1 answer

D is right BUT you have a SERIOUS error in what you wrote. Ksp expression is not what you wrote.
Ksp = (Ca^2+)^3*(PO4^3-)^2. Note that you don't have that. Here is the problem.
Ca3PO4 ==> 3Ca^2+ + 2PO4^3-
I...........0........0
C...........3S.......2S
E..........3S........2S

So Ksp = what I wrote above.
Ksp = (3S)^3(2S)^2 = 108S^5

Note that if you try it your way it is
(3*3S)^3(2*2S)^2 = not 108S^5