An experiment suitable for use in teaching principles of stoichiometry and chemical laboratory techniques is described. It is intended for high school or college courses in general chemistry. The experiment involves dissolving a known mass of copper metal and quantitatively precipitating the copper as copper iodide, cui. A student using the experiment is given as background information the atomic masses of copper and iodine and is expected to determine the empirical formula of copper iodide, that is, to decide from his or her measurement of its I:cu atom ratio whether copper iodide is cui or cui2. In use by a group of typical students at the University of Nevada at Reno, the experiment gave an average result for the I:cu ratio equal to 1.02 With a standard deviation of 0.16.