Solubility Equilibria

 

52) (a) [Co(NO3)2] = 0.020 M          pH = 8.5          Ksp = 1.3 x 10-15           

 

            Co(NO3)2(aq) Co2+(aq) + 2NO3-(aq)

            [Co2+] = 0.020 mol Co(NO3)2/L x 1 mol Co2+/1 mol Co(NO3)2 = 0.020 M

 

            pH + pOH = 14.00

            pOH = 14.00 - pH = 14.00 - 8.5 = 5.5

            [OH-] = 10-pOH = 10-5.5 = 3 x 10-6 M

 

            Co(OH)2(s) Co 2+(aq)   +   2OH-(aq)  

             Q = [Co 2+] x [OH-]2

             Q = 0.020 x (3 x 10-6)2 = 2 x 10-13

 

             Q > Ksp, therefore, Co(OH)2 will precipitate.

 

       (b) [AgNO3] = 0.010 M          [NaIO3] = 0.015 M          Ksp = 3.1 x 10-8

            VAgNO3 = 100. mL             VNaIO3 = 10. mL

 

             AgNO3(s) Ag+(aq) + NO3-(aq)

             NaIO3(s) Na +(aq) + IO3-(aq)

 

             [AgNO3] = n/V

             nAg+ = 0.010 mol AgNO3/L x 1 mol Ag+/1 mol AgNO3 x 100. mL x 1 L/103 mL = 1.0 x 10-3 mol Ag+

             nIO3- = 0.015 mol NaIO3/L x 1 mol IO3-/1 mol NaIO3 x 10. mL x 1 L/103 mL = 1.5 x 10-4 mol IO3-

 

             [Ag+] = 1.0 x 10-3 mol Ag+/(110. mL x 1 L/103 mL) = 9.1 x 10-3 M

             [IO3-] = 1.5 x 10-4 mol IO3-/(110. mL x 1 L/103 mL) = 1.4 x 10-3 M

       

             AgIO3(s) Ag +(aq)   +   IO3-(aq)

             Q = [Ag +] x [IO3-]

             Q = 9.1 x 10-3 x 1.4 x 10-3 = 1.3 x 10-5

 

             Q > Ksp, therefore, AgIO3 will precipitate.