Question

a solution contains 0.050 0 M Ca2+ and 0.030 0 M Ag+. Can 99% of Ca2+ be precipitated by sulfate without precipitating Ag+? What will be the concentration of Ca2+ when Ag₂SO₄ begins to precipitate?

Answer 1

It is not possible to precipitate 99 % of the Ca(2+) without precipitating of Ag.There will also precipitating some of the Ag. In this question solubility product of CaSO4 and Ag2SO4 is not given

Ksp CaSO4= 2.4*10−5
Ksp Ag2SO4= 1.5*10−5

CaSO4(s)⇌Ca(2+)(aq)+SO4(2-)(aq)

Ksp=[Ca(2+)]*[SO4(2-)]=2.4×10^-5

Ag2SO4(s)⇌2Ag⁺ (aq)+SO4^2-(aq)

Ksp=[Ag+]²*[SO2-4]=1.5×10^-5

If we precipitate 99 % of the Ca2+, we have 1 % of the original calcium remaining.

0.01×0.050mol/L=0.0005 mol/L

[Ca2+][SO4(2-)]=2.4×10-5

[SO4(2-)]=2.4×10-5/ [Ca2+]=2.4×10-5/0.0005=4.8×10-2

So, [SO4(2-)] must equal 4.8×10-2 mol/L to precipitate 99 % of the Ca2+.

But Qsp=[Ag+]²[SO4(2-)]=(0.030)^2×(4.8×10-2)=4.32×10-5

Qsp>Ksp

Thus, some silver ion will precipitate.