Example. The solubility of AgCl in water is 0.000013 mole
liter-1 at 25C. What is the solubility-product constant,
Ksp?
Solution. Since one mole of AgCI produces one mole
each of Ag+ and Cl-, the concentration of
each ion in a saturated solution is
Hence,
Example. Would AgCl be more, or less, soluble in 0.01-molar
sodium chloride than in pure water? What would its solubility be?
Solution. The solubility-constant expression tells
us that the product of concentrations of Ag+ and Cl-
is fixed, no matter what the source of the ions. Hence, if we let
x be the solubility of AgCl under these conditions, there will be
0.01 mole of Cl- ion per liter from NaCI, and x moles
per liter from AgCl. The total ion concentrations will be
The solubility-product constant will be the same as in the previous
example; thus,
We can guess in advance that adding more chloride ion will make
AgCl less soluble, because the product of [Ag+] and [Cl-]
remains unchanged, or that x will turn out to be less than 1.3 x
10-5 mole liter-1. We then can neglect x in
comparison with 0.01 in the sum, and thereby avoid having to solve
a quadratic equation. With this simplification,
If you compare this result with that of the preceding example,
you will find that the solubility of AgCl in 0.01-molar NaCI is
only one 76th its solubility in pure water.
This decrease in solubility of a slightly soluble salt in the presence
of another salt that shares an ion is known as the common ion
effect. We could have predicted it from Le Chatelier's principle:
When a system at equilibrium is subjected to a stress, the equilibrium
shifts in such a way as to partially relieve that stress.