How do you find the Ksp when given the concentration of an ion in solution?

Question

The concentration of Ag in a solution saturated with Ag2C2O4(s) is 2.2E-4 M. Calculate Ksp for Ag2C2O4.

Answer 1

Ag2C2O4 <-----> 2 Ag+ + C2O42-

[Ag+] = 2.2 x 10^-4

[C2O42-] = 2.2 x 10^-4 / 2 = 1.1 x 10^-4

Ksp = [Ag+]^2[C2O42-] = ( 2.2 x 10^-4)^2 ( 1.1 x 10^-4) = 5.3 x 10^-12

Answer 2

If [Ag+] = 2.2x10^-4, then [C2O4-2] = (1/2) x 2.2x10^-4 = 1.1x10^-4.

Ksp = [Ag+]^2[C2O4-2]

Ksp = (2.2x10^-4)^2 * (1.1x10^-4) = 5.3x10^-12