Untitled - Kelly Walsh High School

238 CHEMISTRY FOR THE UTTERLY CONFUSED
If the acid being titrated is a weak acid, then there are equilibria that will be
established and accounted for in the calculations. (See the Utterly Confused
section at the end of the chapter.) Typically, a plot of pH of the weak acid solution
being titrated versus the volume of the strong base added (the titrant)
starts at a low pH and gradually rises until close to the equivalence point in
which the curve rises dramatically. After the equivalence point region, the curve
returns to a gradual increase. We can see this in Figure 16-1.
Figure 16-1 Titration of a weak acid with a strong base
16-4 Solubility Equilibria (K sp)
Many salts are soluble in water, but others are only slightly soluble. These salts,
when placed in water, quickly reach their solubility limit and the ions establish
an equilibrium system:
PbSO 4(s) K Pb 2 (aq) SO 4 2 (aq)
The equilibrium constant expression associated with systems of slightly soluble
salts is the solubility product constant, K sp. It is the product of the ionic concentrations,
each one raised to the power of the coefficient in the balanced
chemical equation. It contains no denominator since the concentration of a
solid is, by convention, 1, and for this reason it does not appear in the equilibrium
constant expression. The K sp expression for the PbSO 4 system is:
K sp [Pb 2 ] [SO 4 2 ]