Question

Suppose you have a solution that contains 0.0420 M Ca2 and 0.0960 M Ag . If solid Na3PO4 is added to this mixture, which of the following phosphate species would precipitate out of solution first?

When the second cation just starts to precipitate, what percentage of the first cation remains in solution?

Answer 1

The first step is to search in the literature for the Ksp values

Ksp Ca₃(PO₄)_{2 =} 2 x 10^-29

Ksp Ag₃PO_{4 =} 8.89 x 10^-18

The first specie in precipitate out is those with the smaller value of Ksp. So the first specie in precipitate is Ca₃(PO₄)₂ (Ca²⁺).

The Ksp equilibrium is always true, so for the Ca²⁺ it is true that:

[Ca⁺²][PO3^-3]= 2 x 10^-29

The amount of Ca2+ in solution is X. X = 2 x 10^-29 X²  =2x10^-29  X = (2x10^{-29) 1/2 =}

[Ca2+] =4.47x10^-15

% Ca+2 in solution = (4.47x10^-15 /0.042) x100 = 1.064 x 10^-11 %