Question

Selective Precipitation

Given: Ksp(Ag2CrO4) = 1.2×10-12 Ksp(BaCrO4) = 2.1×10-10

For a solution which is initially 0.003 M in Ba2+ and 0.003 M in Ag+, it is desired to precipitate one of these ions as its chromate (CrO42−) salt to the maximum possible degree without precipitating any of the other cation. From this solution, which ion can be selectively first-precipitated by controlled addition of CrO42− and, ideally, what is the maximum possible percentage of that ion which can be exclusively so precipitated? (Assume that there is no change in the total volume of the solution.)

Ag+

Ba2+

= ?%

Answer 1

Calculate the CrO₄2− ion concentration when BaCrO₄ first begins to precipitate

Ksp = [Ba²⁺] [CrO₄2¯]

2.1×10^-10 = (0.003) [CrO₄2¯]

[CrO₄2¯] = 2.1×10^-10 / (0.003)

[CrO₄2¯] = 7 x 10^-8

Calculate the CrO₄2− ion concentration when Ag₂CrO4 first begins to precipitate

Ksp = [Ag⁺]² [CrO₄2¯]

1.2 ×10^-12 = (0.003)^2 [CrO₄2¯]

[CrO₄2¯] = 1.2 ×10^-12 / (0.003)^2

[CrO₄2¯] = 1.33 x 10^-7

BaCrO₄ precipitates first, because CrO₄2¯ concentration necessary to form BaCrO₄ is smaller.

Calculate the [Ba²⁺] ion when Ag₂CrO4 first begins to precipitate

Ksp = [Ba²⁺] [CrO₄2¯]

2.1×10^-10 = [Ba²⁺][1.33 x 10^-7]

[Ba²⁺] = 2.1×10^-10 /1.33 x 10^-7

[Ba²⁺] = 1.58 x 10^-3

Calculate the percent Ba²⁺ remaining in solution:

(1.58 x 10^-3)/ (0.003) x 100 = 52.67%

The maximum possible percentage of Ba²⁺ ion which can be exclusively precipitated

= 100 – 52.67 = 47.33%