Question

Write the equilibrium expressions for the dissolution of the following insoluble compounds and calculate the molar solubilities of each compound:

a) AlAsO4, Ksp = 1.6x10-16

b) Cu2S, Ksp = 1.6x10-48

c) Pb3(AsO4)2, Ksp = 4.1x10-36

Answer 1

a) AlAsO4, Ksp = 1.6x10-16

    AlAsO4(s) <------------------> Al^3+ (aq) + AsO4^3- (aq)

                                                  s                      s

Ksp            = [Al^3+][AsO4^3-]

1.6*10^-16   = s*s

s^2            = 1.6*10^-16

S              = 1.26*10^-8 M

solubility of AlASO4 is 1.26*10^-8 M

b) Cu2S, Ksp = 1.6x10-48

Cu2S(s) ------------------> 2Cu^+ (aq) + S^2- (aq)

                                          2s                   s

      Ksp   = [Cu^+]^2[S^2-]

   1.6*10^-48   = (2s)^2 *s

   4s^3            = 1.6*10^-48

    s^3             = 0.4*10^-48

    s                = 7.4*10^-17 M

solubility of Cu2S is 7.4*10^-17 M

c) Pb3(AsO4)2, Ksp = 4.1x10-36

    Pb3(AsO4)2 (s) ----------------------> 3Pb^2+ (aq) + 2AsO4^3- (aq)

                                                                3s                   2s

   Ksp   = [Pb^2+]^3 *[AsO4^3-]^2

4.1*10^-36 = (3s)^3 *(2s)^2

108s^5        = 4.1*10^-36

     s^5          = 0.0379*10^-36

    s              = 5.2*10^-9 M

solubility of Pb3(asO4)2 is 5.2*10^-36 M