Question

Sodium sulfate is slowly added to a solution containing 0.0500 M Ca2 (aq) and 0.0320 M Ag (aq). What will be the concentration of Ca2 (aq) when Ag2SO4(s) begins to precipitate? Solubility-product constants, Ksp, can be found here. What percentage of the Ca2 (aq) can be precipitated from the Ag (aq) by selective precipitation?

Answer 1

Ag2SO4 (s)   -----------> 2 Ag+ (aq) + SO42- (aq)

[Ag+] = 0.0320 M

when Ag2SO4 precipitate :

Ksp = [Ag+]^2[SO42-]

1.20 x 10^-5 = (0.0320)^2 [SO42-]

[SO42-] = 0.0117 M

CaSO4   ------------> Ca2+   +   SO42-

Ksp = [Ca2+] [SO42-]

4.93 x 10^-5 = [Ca2+] (0.0117)

[Ca2+] = 4.21 x 10^-3 M

concentration of Ca2 (aq) = 4.21 x 10^-3 M

b)

% of Ca2+ = 4.21 x 10^-3 / 0.0500 ) x 100

                 = 8.4 %

% of the Ca2 (aq) can be precipitated = 91.6 %