Solubility Equilibria

 

56) (a) [Ba2+] = 0.0150 M          Ksp(BaSO4) = 1.1 x 10-10

            [Sr2+] = 0.0150 M          Ksp(SrSO4) = 3.2 x 10-7

 

            BaSO4(s) Ba2+(aq) + SO42-(aq)

            Ksp = [Ba2+] [SO42-]

            1.1 x 10-10 = 0.0150 x [SO42-]

            [SO42-] = 7.3 x 10-9 M

 

            SrSO4(s) Sr2+(aq) + SO42-(aq)

            Ksp = [Sr2+] [SO42-]

            3.2 x 10-7 = 0.0150 x [SO42-]

            [SO42-] = 2.1 x 10-5 M

 

            Precipitation begins when Q = Ksp and when [SO42-] = 7.3 x 10-9 M because it is the smaller of the two values.

             

      (b) Ba2+ precipitates first because it requires the least SO42-.

 

      (c) Sr2+ begins to precipitate when [SO42-] = 2.1 x 10-5 M.