Question

A solution contains 0.0020 M concentrations of I- and CO3^2-. What is [CO3^2-] when PbI2 just starts to precipitate? For PbI2, Ksp= 1.4x10^-8, and for PbCO3, Ksp= 1.4x10^-13.

Answer 1

Solution:

• Concentration of iodine ion, [I-​​​​​​] = 0.002 M
• Concentration of carbonate ion, [CO32-] = 0.002 M
• Solubility product of PbI2, Ksp = 1.4 x 10-8​​​​​​
• Solubility product of PbCo3, Ksp = 1.4 x 10-13

Ksp(PbI2) = [I-​​​​​​]2 x [Pb2+]

1.4 x 10-8 = (0.002)2 x [Pb2+]

[Pb2+] = 0.0035 M

When PBI2 starts precipitating, the concentration of CO32- is given by

[CO32-] = 4 x 10-11 M