A solution contains 0.025 M Ca2+ and 0.015 M Ag+. Determine whether it is possible to precipitate 99% of Ca2+ by sulphate without precipitating Ag+?

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

1 Answer
Ernest Z.
Dec 12, 2017

It is not possible to precipitate 99 % of the Ca2+ without also precipitating some of the Ag+.

Explanation:

Step 1. Write the equations for the equilibria

CaSO4(s)⇌Ca2+(aq)+SO2-4(aq)

Ksp=[Ca2+][SO2-4]=2.4×10-5

Ag2SO4(s)⇌2Ag+(aq)+SO2-4(aq)

Ksp=[Ag+]2[SO2-4]=1.5×10-5

Step 2. Calculate the sulfate concentration needed to precipitate the calcium

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

0.01×0.025 mol/L=2.5×10-4lmol/L

[Ca2+][SO2-4]=2.4×10-5

[SO2-4]=2.4×10-5[Ca2+]=2.4×10-52.5×10-4=9.6×10-2

So, [SO2-4] must equal 9.6×10-2lmol/L tp precipitate 99 % of the Ca2+.

Step 3. Will this precipitate any of the silver?

Qsp=[Ag+]2[SO2-4]=(0.015)2×9.6×10-2=2.2×10-5

Qsp>Ksp

Thus, some silver ion will precipitate.

It is not possible to precipitate 99 % of the Ca2+ without also precipitating some of the Ag+.

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