Question

Solubility of   Solids
6. What   would   the   calcium   concentration   be   in   a   saturated   CaSO4 solution?
7. What   would   the   calcium   concentration   be   in   a   saturated   Ca3(PO4)2 solution?

Answer 1

6)
Ksp of CaSO4 = 2.4*10^-5
calcium concentration keeps increasing untill maximum solubility and after than it forms precipitate. Let this maximum solubility be s. The solution becomes saturated when concentration is maximum
So we need to just calculate s

CaSO4 <-----> Ca2+ + SO42-
s s

Ksp = [Ca2+][SO42-]
2.4*10^-5 = s*s
s = 4.9*10^-3 M
Answer: 4.9*10^-3 M

7)
ksp of Ca3(PO4)2 = 1.3*10^-32
Ca3(PO4)2 ----> 3 Ca2+ + 2 PO4 3-
3s 2s
Ksp = (3s)^3 (2s)^2
1.3*10^-32 = 108*s^5
s^5=1.2037*10^-34
s = 1.64*10^-7 M
Answer: 1.64*10^-7 M