A general chemistry laboratory experiment is described that introduces students to instrumental analysis using gas chromatography-mass spectrometry (GC-MS), while simultaneously reinforcing the concepts of mass percent and the calculation of atomic mass. Working in small groups, students use the GC to separate and quantify the percent composition in a mixture of dichloromethane and chloroform dissolved in toluene by the injection of known quantities of each pure substance dissolved in toluene and the determination of the individual instrumental response factor. The relative abundances of the chlorine-35 and chlorine-37 isotopes can also be determined from several different groupings on the mass spectra of the compounds, allowing for the calculation of the atomic mass of chlorine. This collaborative-learning experiment enhances student understanding through the hands-on use of chemical instrumentation and helps to develop their problem solving skills in the analysis of several different types of experimental data. (Contains 3 tables and 3 figures.)