Some chemical concepts, such as limiting reagents, seem to be difficult for some students to grasp. A new laboratory experiment is described in which Job's method is applied to the gravimetric determination of the formula of a sparingly soluble ionic compound. The formula of the compound depends on whether the medium is acidic, neutral, or basic, making it possible to assign an �unknown� to each student. The plot of the mass of the precipitate versus the mole fraction of the cation yields the formula. Students perform calculations to determine the limiting reagent that controls the formation of each precipitate. This experiment also demonstrates the concept of stoichiometric relationships between ions in the formation of a compound, and gives students experience in using a spreadsheet to manipulate data. The experiment is suitable for use in a college general chemistry curriculum or in a high school AP or honors chemistry class.

Using an innovative experiment to teach a “difficult” concept

243rd National Meeting (2012)

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