Question

Concentrated NaF(aq) is slowly added to a solution that is 0.010 M CaCl2 and 0.010 M MgCl2.

a. What is the first precipitate to form?

b. What [F-] is needed to start precipitating the second precipitate?

c. Can Ca2+ and Mg2+ be separated by selective precipitation of their fluorides? Explain.

Answer 1

For this you need the Ksp values of both CaF2 and MgF2
From wikepedia,
CaF2: 3.9 x 10^-11
MgF2: 5.16·10^-11
I assume you know Ksp=(Ca2+) (F-)^2. and same for MgF2.
So now apply the formula Ca2+=Mg2+=0.01.
Hence F- needed to precipitate CaF2=3.9*10^-9 and for MgF2 it is 5.16*10^-9. Clearly CaF2 requires less F- concentration and precipitates first.
Concentration of F- needed for precipitation of MgF2 is 5.16*10^-9.
You cannot use fluoride for selective precipitation as their Ksp values are very similar.