Two or More Precipitate Ions in a Single Solution
When a precipitating agent is slowly added to a solution that contains more ions than precipitates, then the compound whose solubility product is exceeded first will precipitate first. If I-1 ion solution is slowly added to a solution containing 0.010 mol/L Ag+1  and 0.010 mol/L Pb2+, AgI precipitates as soon as the [I-1] exceeds:
 
            Ksp = [Ag+][I-]
8.3 x 10-17 = (1.0 x 10-2)[I-]
             [I-] = 8.3 x 10-15 mol/L
 
The lead ions do not precipitate until [I-] exceeds

Ksp = [Pb2+][I-]2 since PbI2 <=====> Pb2+ + 2 I-

[I-]2    =   Ksp   =   7.9 x 10-9
              [Pb2+]     1.0 x 10-2

[I-]2 = 7.9 x 10-7


 

Follow-Up Problem
A solution contains 0.01 mol/L Ag+1 ions and 0.01 mol/L Sr2+ ions. Which ion precipitates first, when K2CrO4 is slowly added to the mixture?