Question

Assuming that no equilibria other than dissolution are involved, calculate the concentration of all solute species in each of the following solutions of salts in contact with a solution containing a common ion. Show that changes in the initial concentrations of the common ions can be neglected.

a)  PbI2(s) in 0.0355 M CaI2

b) Ag2CrO4(s) in 0.225 L of a solution containing 0.856 g of K2CrO4

Answer 1

PbI2 = PB+2 and 2I-

Ksp = [Pb+2][I-]^2

if CaI2 = 0.0355

then I- = 2*0.0355 = 0.071

then

Ksp = [Pb+2][I-]^2

1.38*10^-8 = S *(0.071^2)

S = (1.38*10^-8)/(0.071^2) = 0.00000273755 for PbI2

b)

Mw of K2CrO4 = 194.19

mol = mass/MW = 0.856/194.19 = 0.00440805 mol

then calculate

M = mol/V = 0.00440805/(0.225) =0.019591

[CrO4-2] = 0.019591

for

Ag2CrO4 = 2Ag+ + CrO4-2

Ksp = [Ag+]^2[CrO4-2]

9*10^-12 = (S)^2 * (0.019591)

S = sqrt((9*10^-12)/(0.019591))

S= 0.00002143349 M fo Ag2CrO4