Cliffs AP Chemistry, 3rd Edition

Experiment 10 B: Determination of the Equilibrium Constant, Ksp, for a Chemical Reaction

Background: Lead iodide is relatively insoluble, with a solubility of less than 0.002 M at
20 °C. Lead iodide dissolved in water is represented as:

PbI 2 ()sEPb()^2 aq+-+ 2 I()aq

The solubility product expression (Ksp) for this reaction is:

KPbIsp^2

2

= 77 +-AA

If lead iodide is in a state of equilibrium in solution, the product of the molar concentration of
lead ions times the square of the molar concentration of the iodide ions will be equal to a con-
stant. This constant does not depend upon how the state of equilibrium was achieved.

When standard solutions of lead nitrate, Pb(NO 3 ) 2 and potassium iodide, KI, are mixed in the
presence of KNO 3 , a yellow precipitate of lead (II) iodide, PbI 2 , forms. The potassium nitrate
helps to keep the solution at a nearly constant ionic molarity and promotes the formation of
well-defined crystals of lead iodide. The lead iodide will be allowed to come to equilibrium in
the solution by mixing the solution very well and then allowing the precipitate to settle com-
pletely followed by centrifuging. The iodide ion, I–, is colorless and not able to be directly mea-
sured in a spectrophotometer (see Figure 1). Therefore, the I–will be oxidized to I 2 which is
brown in water by an acidified potassium nitrite, KNO 2 , solution. The concentration of I 2 is
rather small, however, the absorption of light at 525 nm is sufficient to allow detection of the I 2
molecule. [Pb2+] in solution will be determined from the initial state of the system, the mea-
sured [I–] and the relationship between [Pb+2] and [I–] in the equilibrium expression.

Figure 1

meter wavelength scale

sample holder power switch

and zero knob

light control

knob wavelength

control knob

Laboratory Experiments