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

Question

0.10 M potassium chromate is slowly added to a solution containing 0.20 M AgNO3 and 0.20 M Ba(NO3)2. What is the Ag+ concentration when BaCrO4 just starts to precipitate? Ksp for Ag2CrO4 and BaCrO4 are 1.1 × 10-12 and 1.2 × 10-10, respectively.

a. 4.3 × 10-2 M

b. 1.3 × 10-4 M

c. 3.2 × 10-4 M

d. 6.5 × 10-5 M

Answer 1

K2CrO4 ------- 2K+   +  CrO42-

Conc CrO4^2- = 0.1M

Ba(NO3)2  -------  Ba2+  +  2NO3-

conc Ba2+ = 0.2M

BaCrO4 ---------Ba2+  +  CrO4^2-

Kps BaCrO4 = 1.2x10^-10

Kps = [Ba2+][CrO4^2-]

Conc CrO4^2- =  Kps/0.2 = 6x10^-10

Ag2CrO4--------  2Ag+  +  CrO4^2-

concCrO4^2- =6x10^-10

Kps Ag2CrO4 = 1.1x10^-12

Conc Ag+  =  sqrt (Kps /6x10^-10)= 4.3x10-2M