# Homework Help: Calculating Ksp of salt (answer check)

1. Apr 25, 2006

### ScrubsFan

69. Calculate the Ksp for each of the salts whose solubility is listed below.

a) CaSO4 = 3.3 x 10-3 mol/L
b) MgF2 = 2.7 x 10-3 mol/L

a) Solution

1. Write the dissociation reaction and the Ksp expression for the equilibrium reaction. Thus,

CaSO4(s) <---> Ca2+ (aq) + SO42- (aq)

Ksp = [Ca2+][SO42-]

2. Calculate the concentrations of the ions at equilibrium from the stoichiometric information provided in the equilibrium equation. The equation shows that 1 mol of CaSO4 produces 1 mol of Ca2+ and 1 mol of SO42- at equilibrium. Therefore,

[Ca2+] = 3.3 x 10-3 mol/L
[ SO42-] = 3.3 x 10-3 mol/L

3. Substitute the concentrations into the Ksp expression:

Ksp = (3.3 x 10-3)( 3.3 x 10-3)

= 1.2 x 10-5

b) MgF2 <---> Mg2+ (aq) + F22+ (aq)

Ksp = [Mg2+][F22+]

[Mg2+] = 2.7 x 10-3
[F22+] = 2.7 x 10-3
Ksp = (2.7 x 10-3)(2.7 x 10-3)

= 7.3 x 10-6

2. Apr 25, 2006

### andrewchang

your first one looks right, but the F2 in the second equation will actually dissociate into two F- ions

3. Apr 25, 2006

### ScrubsFan

Alright I fixed part b), is it right now?

MgF2 <---> Mg2+ (aq) + 2F- (aq)

Ksp = [Mg2+][F-]2

[Mg2+] = 2.7 x 10-3
[F-] = 2 x 2.7 x 10-3
Ksp = (2.7 x 10-3)(5.4 x 10-3)

= 1.5 x 10-5

Last edited: Apr 26, 2006