M NaCl and 10-3 M K2CrOd Individual solutions of can he used to verify calculations for the volume of 0.1 M AgN03 necessary to form a precipitate. A laser beam passing through the solutions enables the students to observe the first formation of a colloidal precipitate via the Tyndall effect. A mixed solution containing 10-3M NaCl and M KpCr04 can then he used to verify that chloride may he completely separated from chromate via precipitation. An element of charm is introduced when the red AgpCrOa precipitates hefore the AgCI, only to dissolve in a few seconds. Resulting mixtures can then he used to determine how addition of H+ and NH3affect precipitate solubility. Thus, quantitative and qualitative predictions can he made and verified in approximately 0.5 h of class time. Observation of the laser beam scattering is easiest in a small classroom. The demonstration is useful in a large lecture hall, however. In addition, the Ag2Cr04 precipitate formation portion of the demonstration can he used separately as an experimental determination of the AgpCrOa solubility product constant.
10 mL 8 M HN03 10 mL 1 M NH3 10-mL graduated cylinder disposable Pasteur pipet and bulb or micropipet magnetic stir plate and three stir bars He-Ne laser or bright lamp and clamps to position (optional)
Experiment
Joseph H. Drelsbach Unlrerslty o( Scranton
First work through the calculations todetermine the minimum concentration of Ag+ and the minimum volume of 0.1 M AgNOl necessary to begin precipitation when the AgN03 isadded to500 mLof 1.0 X 10." NaCl and toS00mLof 1.0 X 10-:'M K2CrO4ll.9X 10-:M,0.001 mLforCIk4.1 X M, 0.21 mL for CrOa2-). Place the heaker of KzCrOl on the stir plate with the laser beam uassine verticallv throueh the solution. (Caution: Have students close their eyes before turning on laser. Verify that reflections do not go toward the audience.) Add AgN03 dropwise with a Pasteur pipet (0.03 mL/drop) or a micropipet from 5 mL of solution in a graduated cylinder. A red precipitate will form and dissolve with each drop. The permanent preci~itatecan he observed by scattered light after 7 drops ( 6 9 drops theoretical). In the absence of a laser, a 1 0 dropsmight he necessary for alarge class to heahle tosee the permanent precipitate. Add the remainder of the AgN03, and set the beaker aside. Do the same with the NaCl solution. The theoretical value is
