A Closer Look at the Common Molar Mass Experiment

The “Molar Mass (or Molecular Weight) of a Volatile Liquid” laboratory is a common chemistry experiment performed by many school students, as well as college students. Usually, a “simple” variant of the Dumas bulb method is applied to determine the molar masses of volatile liquids, applying the ideal gas law. The evaluation of this classic experiment starts with a theoretical analysis of the experimental conditions. Shortcomings of the experiment’s underlying model are pointed out. A corrected model is developed targeting to bring achievable experimental results closer to “reality”. A simulation is performed, revealing how severe results deviate when using the classic experiment in the classroom, important information for educators. Experimental data confirming the theoretical considerations and simulations are provided. The results presented provide a tool to handle the common discrepancies associated with the classic Molar Mass experiment and, therefore, present a valuable teaching resource. An important aspect of this publication is to demonstrate that basic and common chemistry experiments provide a variety of science teaching opportunities. Throughout this experiment’s evaluation, instructor targeted suggestions are provided to highlight teaching opportunities at levels ranging from middle school to graduate level chemistry.