Module 7.3: Solubility

I Solubility product ($\textrm{K}_{sp}$): the equilibrium constant for the equilibrium between an ionic solid and its ions

1. determines the extent of solubility for a certain ionic compound. For example, $\textrm{PbS}$ is:

   $\textrm{PbS}(\textrm{s}) \leftrightharpoons \textrm{Pb}^{2+}(\textrm{aq}) + \textrm{S}^{2-}(\textrm{aq})$ $\textrm{K}_{sp} = [\textrm{Pb}^{2+}][\textrm{S}^{2-}]$

2. note that these equilibria are only meaningful for sparingly soluble salts as:

   1. ionic solids with strong ionic interactions exist as ion pairs even in water

   2. these ionic solids may form clusters in water, not being separated in water

3. calculating the molar solubility of a salt is convenient with $\textrm{K}_{sp}$.

II common-ion effect: the decrease in solubility due to the addition of an ion present in the salt

1. A consequence of the Le Chatelier's principle

2. difficult to predict the common-ion effect accurately due to strong ion-ion interaction

III $\textrm{Q}_{sp}$ (reaction quotient) allows you to predict whether mixing two ions will form a precipitate:

1. When $\textrm{Q}_{sp}$ is larger than $\textrm{K}_{sp}$, the salt will precipitate.

2. When $\textrm{Q}_{sp}$ is less than $\textrm{K}_{sp}$, the salt will dissolve.