Question

What is the solubility in gram/liter for CaF2 (Ksp = 3.9 x 10-11)

Answer 1

The salt dissolves as:

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

   s 2s

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

3.9*10^-11=(s)*(2s)^2

3.9*10^-11= 4(s)^3

s = 2.136*10^-4 M

Molar mass of CaF2 = 1*MM(Ca) + 2*MM(F)

= 1*40.08 + 2*19.0

= 78.08 g/mol

Molar mass of CaF2= 78.08 g/mol

s = 2.136*10^-4 mol/L

To covert it to g/L, multiply it by molar mass

s = 2.136*10^-4 mol/L * 78.08 g/mol

s = 1.67*10^-2 g/L

Answer: 1.67*10^-2 g/L