Potassium chromate is slowly added to a solution containing 0.20 M AgNO3 and 0.20 M Ba(NO3)2.?

Question

Describe what happens if the Ksp for Ag2CrO4 is 1.1x10^(-12) and teh Ksp of BaCrO4 is 1.2x10^(-10).

a. The Ag2CrO4 precipitates first out of the solution and then BaCrO4 precipitates

b. The BaCrO4 precipitates first out of the solution

c. Both BaCrO4 and Ag2CrO4 precipitate simultaneously out of the solution

d. Neither BaCrO4 nor Ag2CrO4 precipitates ou of the solution

Answer 1

Ignore dilution effect of added chromate solution.  Let c be the concentration of chromate and which precipitation begins.

To begin precipitation of Ag2CrO4, c x (0.2)^2 = 1.1 x 10^-12; c = 2.75 x 10^-11.

Very similarly, BaCrO4 begins to precipitate when c = 6 x 10^-10.

A puzzle for you; how much Agplus remains in solution when barium chromate begins to precipitate?

Answer 2

(a) is correct.

As the Ksp of the silver salt is the smaller of the two it will be exceeded first.  As soon as the value is exceeded solid silver chromate will separate from the solution.  As more K2CrO4 is added more silver chromate will precipitate, maintaining the constancy of the Ksp.

The concentration of silver ions will have to be very small before any barium salt starts to form.