What are some common mistakes students make with solubility equilibria?

1 Answer
Nov 27, 2015

Many students fail to realize that the precipitate is irrelevant.

Explanation:

For an insoluble salt, MX, usually a solubility product, Ksp, at some particular temperature, we can write the normal equilibrium expression:

MX(s)M+(aq)+X(aq)

As for any equilibrium, we can write the equilibrium expression,

[M+(aq)][X(aq)]MX(s) = Ksp.

Now normally, we have some handle on [X] or [M+], but the concentration of the solid material [MX(s)] is meaningless and irrelevant; it is arbitrarily treated as 1. So,

[M+(aq)][X(aq)] = Ksp.

There may often be a precipitate of MX(s) in the bottom of the flask, however, this is completely irrelevant to the solubility product, and to the equilibrium. It is out of the game as a precipitate. [X], may be artificially raised to some extent as well (i.e. by introducing beforehand a soluble salt of X; such a procedure is called "salting out"). If M was a precious metal (say gold or rhodium or iridium), you would want to precipitate all this is out as an insoluble salt, as opposed to washing it down the sink.