H 2 O
BaSO 4 (s)3::4Ba^2 (aq)SO 42 (aq)(We usually omit H 2 O over the arrows in equations like the preceding one.)
In equilibria that involve slightly soluble compounds in water, the equilibrium constant
is called a solubility product constant, Ksp.The activity of the solid BaSO 4 is one (Section
17-11, Heterogeneous Equilibrium). Hence, the concentration of the solid is not included
in the equilibrium constant expression. For a saturated solution of BaSO 4 in contact with
solid BaSO 4 , we writeBaSO 4 (s) 34 Ba^2 (aq)SO 42 (aq) and Ksp[Ba^2 ][SO 42 ]The solubility product constant for BaSO 4 is the product of the concentrations of its
constituent ions in a saturated solution.In general, the solubility product expressionfor a compound is the product of
the concentrations of its constituent ions, each raised to the power that corresponds
to the number of ions in one formula unit of the compound. The quantity is constant
at constant temperature for a saturated solution of the compound. This statement
is the solubility product principle.The existence of a substance in the solid state is indicated several ways. For example,
BaSO 4 (s), BaSO 4 , and BaSO 4 gare sometimes used to represent solid BaSO 4. In this text
we use the (s) notation for formulas of solid substances in equilibrium with their saturated
aqueous solutions.Barium sulfate is the “insoluble”
substance taken orally before stomach
X-rays are made because the barium
atoms absorb X-rays well. Even though
barium ions are quite toxic, barium
sulfate can still be taken orally without
danger. The compound is so insoluble
that it passes through the digestive
system essentially unchanged.
824 CHAPTER 20: Ionic Equilibria III: The Solubility Product Principle
The dissolving process of ions going
into solution. During precipitation,
ions are producing more solid as
the ions leave the solution. For a
saturated solution, the dissolving
and precipitation rates are equal. For
the very slightly soluble salts
discussed in this chapter, the ion
concentrations are very low
compared with the concentration
in this illustration.
An X-ray photo of the gastro-
intestinal tract. The barium ions in
BaSO 4 absorb X-radiation well.