# K from Experiment

Learning Goals

You will learn how to calculate the Ksp from experimental measurements of ion concentrations.

Synopsis

In calculating the Ksp from experimental ion concentrations, you must be careful to use the stoichiometric coefficients properly. For the hypothetical salt AaBb, the equilibrium is AaBb --> aA+ + bB- and you know that Ksp = [A+]a[B-]b. So if you are given the concentration of one of the ions in solution, how do you calculate the concentration of the other ion to use in the Ksp solution? If the concentration of the A ion is x, what is the concentration of the B ion? We just use the stoichiometric coefficients carefully and we can easily see that if [A+] = x, then [B-] must be b/a of [A+].

For example, let's calculate the Ksp for BaF2 given that in a saturated solution [Ba2+] is 7.5 x 10-3 M. Because the equilibrium is

BaF2 Ba2+ + 2F-,

[F-] = 2[Ba2+] = 1.50 x 10-2 M so the equilibrium constant is (7.5 x 10-3) (1.50 x 10-2)2 = 1.69 x 10-6.

Review Question

Calculate the equilibrium constant for the salt AuI3 given that the Au3+ ion concentration in a saturated solution is 1.387 x 10-12 M.

Web Author: Dr. Leon L. Combs