Dale 1. Wilhelm Ohio Northern University A ~ O ,45810
The Law of Definite Proportions An experiment for introductory chemistry
Experiments which test the fundamental laws of chemistry-have special value in the introductory laboratory. However, practical limitations of lahoratory time, skill of the student, and quality of lahoratory tools may lead to questionable choices of experiments. An experiment testing the Law of Definite Proportions is found in many lahoratory textbooks. Subject compounds are binary compounds in which simple and reproducible stoichiometry is expected. Copper(1) sulfide is the most frequent choice; magnesium oxide and silver chloride are also used. For various reasons another choice of compound would be desirable. First, the experiment should focus on repeated synthesis of the compound and that repeated synthesis yields a product in which the weight-ratio of the component elements is constant. The available time in a single lahoratory period makes difficult the repetition of a synthesis and determination of composition for most compounds now in use. Further, related to the time requirement is the demand on manipulative skills; some present experiments involve techniques in gravimetric analysis usually well developed late in the first year of college instruction. Finally, in the case of copper(1) sulfide the product obtained under usual laboratory conditions has been found to have indefinite and probably non-reproducihle stoichiometry.' Thus, one can list criteria for a desirable choice of a new system for study. Repeatability in one laboratory period implies a rapid reaction in synthesis and then rapid analysis, that is, rapid isolation of product in easily weighable form. Requisite laboratory skills should he of modest difficulty. Finally, the product should have a well-defined stoichiometry without maintaining rigid synthesis conditions. 'Dingledy, D. and Barnard, W. M., J. CHEM. EDUC., 44, 242 (1967).
436
/Journal of Chemical Education
We have found that the synthesis of copper(1) iodide meets these criteria auite well. C o n ~ e rmetal e x ~ o s e dto iodine vapors reacts i o form an ad
