Question

Can Ni²⁺ be separated from Ag⁺ (to a purity > 99%) by the slow addition of CO₃^2- ? Relevant Ksp are: NiCO₃, 1.4 x 10^-7, Ag₂CO₃, 8.1 x 10^-12. How much of the less soluble ion remains in solution when the second starts to precipitate?

Answer 1

Write down the two dissociation reactions as below:

NiCO₃ (s) <=====> Ni²⁺ (aq) + CO₃^2- (aq)

Let x be the solubility of CO₃^2- in solution; therefore, we have

K_sp = [Ni²⁺][CO₃^2-] = (x).(x)

===> 1.4*10^-7 = x²

===> x = 3.742*10^-4

The solubility of CO₃^2- from NiCO₃ is 3.742*10^-4 M. Therefore, the solubility of NiCO₃ in solution is 3.742*10^-4 M.

Let us look at Ag₂CO₃. The dissociation reaction is

Ag₂CO₃ (s) <======> 2 Ag⁺ (aq) + CO₃^2- (aq)

Let y be the solubility of CO₃^2-; therefore, the solubility of Ag⁺ = 2y. Therefore,

K_sp = [Ag⁺]²[CO₃^2-] = (2y)².(y) = 4y³

===> 8.1*10^-12 = 4y³

===> y³ = 2.025*10^-12

===> y = 1.265*10^-4

The solubility of CO₃^2- from Ag₂CO₃ is 1.265*10^-4 M. Therefore, the solubility of Ag₂CO₃ in solution is 1.265*10^-4 M.

Since the solubility of the two salts are close, hence, it may not be possible to separate both NiCO₃ and Ag₂CO₃ with greater than 99% purity by the addition of CO₃^2- (ans).

For us to predict the amount of the less soluble ion, i.e, Ni²⁺ remaining in solution, we need to know the initial concentrations of both the salts.