K, Made Visible One of the most difficult concepts for a student in Advanced Placement Chemistry to assimilate is solubility products. Most can master the mathematics involved. hut to realize that a comoound havine..a K. .? of 2.6 X 10-l3 is still verv sliehtlv .. . soluble is verv difficult. Some w.;ihlc slan uI rhis limited soluhdity during an enpcrtmcnt dori make the cmcept mow coneretr. A n d - u s e d exprrirnmt to dwelop this concept is one known aa I'rogrcs%ivrI'rrcipitations.' The experiment works well, hut it due3 involve the fcmotntiun of wrnpmndr uf silver. .Many of our strained budgrh will dimply nut atrrreh lo purchase a~lrernitrate. Asr thir reason, a parallel experiment was developed using lead compounds. This is strictly a qualitative experiment, so that exact concentrations and volumes are not necessary. About 5 mL of 0.5 M Ph(NOd2 is put into a test tube along with 10 mL of 0.5 M N&Oc stoppered, and shaken. The resulting PbS04 (white) is allowed t o settle and the supernatant liquid is decanted. Next 10 mL of 1.0 M KI is added to the solid PhS04, stoppered, and shaken. The resulting PbIz (yellow) is settled, and liquid decanted, and the precipitate washed with water and decanted. Next 10 mL of 0.5 M N a ~ C 0 3is added to the solid Pb12,giving the white precipitate of PbC03. Finally, to the solid PbC03 is added 10 mL of either 0.5 M ( N H M or NarS, giving the black PhS. original compound to a subsequent compound. The mechanism that allows this experiment to proceed is the removal of a large amount of one of the ions that are involved in exceeding the K., of a compound. For example, when the supernatant liquid aver the PhIz precipitate is removed, the number of I- ions is greatly reduced. This allows the addition of the NazCOs solution to provide a much larger concentration of CO? ions than the I- ions from the dissociation of Pb12, exceeding theK, of PbC03. The order in which the variouscompounds are formed isdictated mainly by thedesire to have the most striking color changes. -
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example, "Sciemilic Experiment. in Chemishy by Elben C.Weaver. (Ed!forl Holt. Rinehan. and Winston. Inc.. New Yo*.
lSW.
Donald E. Buss Bolingbrook High School Bolinabrook. IL 60439
958
Journal of Chemical Education
