Question

Rank the following salts from least to most molar solubility:

BiI Ksp = 7.7x10-19

Cd3(AsO4)2 Ksp = 2.2x10-33

AlPO4 Ksp = 9.8x10-21

CaSO4 Ksp = 4.9x10-5

I know the answer, although I don't know how

Answer 1

a)

At equilibrium:

BiI <----> Bi+ + I-

   s s

Ksp = [Bi+][I-]

7.7*10^-19=(s)*(s)

7.7*10^-19= 1(s)^2

s = 8.775*10^-10 M

b)

At equilibrium:

Cd3(AsO4)2<----> 3 Cd2+ + AsO43-

   3s s

Ksp = [Cd2+]^3[AsO43-]

2.2*10^-33=(3s)^3*(s)

2.2*10^-33= 27(s)^4

s = 3.004*10^-9 M

c)

At equilibrium:

AlPO4 <----> Al3+ + PO43-

   s s

Ksp = [Al3+][PO43-]

9.8*10^-21=(s)*(s)

9.8*10^-21= 1(s)^2

s = 9.899*10^-11 M

d)

At equilibrium:

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

   s s

Ksp = [Ca2+][SO42-]

4.9*10^-5=(s)*(s)

4.9*10^-5= 1(s)^2

s = 7*10^-3 M

Conclusion:

The compounds in increasing order of solubility are:

AlPO4 whose solubility is 9.899*10^-11 M

BiI whose solubility is 8.775*10^-10 M

Cd3(AsO4)2 whose solubility is 3.004*10^-9 M

CaSO4 whose solubility is 7*10^-3 M