Question

Which of the following has the greatest molar solubility?

Ca3(PO4)2 Ksp = 1.0 x 10-26

CaF2 Ksp = 3.4 x 10-11

CaCO3 Ksp = 3.9 x 10-9

CaC2O4 Ksp = 2.3 x 10-9

CaSO4 Ksp = 2.4 x 10-5

Answer 1

At equilibrium:

Ca3(PO4)2 <----> 3 Ca2+ + 2 PO43-

   3s 2s

Ksp = [Ca2+]^3[PO43-]^2

1*10^-26=(3s)^3*(2s)^2

1*10^-26= 108(s)^5

s = 2.474*10^-6 M

At equilibrium:

CaF2 <----> Ca2+ + 2 F-

   s 2s

Ksp = [Ca2+][F-]^2

3.4*10^-11=(s)*(2s)^2

3.4*10^-11= 4(s)^3

s = 2.041*10^-4 M

At equilibrium:

CaCO3 <----> Ca2+ + CO32-

   s s

Ksp = [Ca2+][CO32-]

3.9*10^-9=(s)*(s)

3.9*10^-9= 1(s)^2

s = 6.245*10^-5 M

At equilibrium:

CaC2O4 <----> Ca2+ + C2O42-

   s s

Ksp = [Ca2+][C2O42-]

2.3*10^-9=(s)*(s)

2.3*10^-9= 1(s)^2

s = 4.796*10^-5 M

At equilibrium:

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

   s s

Ksp = [Ca2+][SO42-]

2.4*10^-5=(s)*(s)

2.4*10^-5= 1(s)^2

s = 4.899*10^-3 M

The compound with maximum solubility is:

CaSO4

Answer: CaSO4