A Simple Experiment for the Determination of Molecular Weights of Gases Lighter Than Air

An experiment for the determination of the molecular weight of gases of low molecular weight is described. The method involves filling a small balloon with the gas to be determined. The weight of the gas inside the balloon is obtained by subtracting the weight of the balloon from the weight of air displaced when the balloon just floats in air, neither rising nor sinking. From the volume of the gas inside the balloon and the weight of the gas calculated, the density and molecular weight of the gas can be calculated. The experiment is simple, low-cost, and easy to perform. It is applicable to gases of low molecular weight such as hydrogen, helium, methane, and natural gas (mostly methane). Typical results are within 1-3% of the true molecular weight of the gas being determined.