In the Laboratory
A Simple Experiment for the Determination of Molecular Weights of Gases Lighter Than Air
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Van T. Lieu* and Gene E. Kalbus Department of Chemistry and Biochemistry, California State University, Long Beach, CA 90840
A simple experiment by Kalbus and Petrucci for the determination of the molecular weight of carbon dioxide has appeared in this Journal (1). The method involves subliming a weighed amount of dry ice inside a balloon. From the volume of carbon dioxide gas inside the balloon and the weight of the dry ice used, the density and molecular weight of carbon dioxide gas were calculated. This method, however, is limited to the determination of the molecular weight of carbon dioxide in the sense that carbon dioxide is the only compound that is readily available as a solid (dry ice) at room temperature and atmospheric pressure and can be easily weighed. In this paper, we have extended the work of Kalbus and Petrucci and describe an experiment for the determination of the molecular weights of gases that are lighter than air. We apply the principle that the weight of a balloon and its gas equals the weight of the air displaced when the balloon just floats in air, neither rising nor sinking. Procedure Inflate a balloon to prestretch its wall, then deflate it. Press all air out of it. Weigh the empty balloon. Inflate the balloon with the gas of interest. Tie the balloon by stretching and making a knot. Loosely attach a string to the balloon. Let the balloon float in air. Adjust the weight of balloon by adding to or cutting off portions of the string so that the balloon neither rises nor sinks. Untie the string and weigh it. Fasten a tape vertically on the outside of a 1000-mL beaker. Collect the gas by puncturing the balloon under water inside the inverted beaker filled with water in a bucket. After equalizing the water levels inside and outside the beaker by raising or lowering the beaker, mark the water level on the tape and record the volume of the gas. A typical experiment yields a volume of about 600 mL. Record the barometric pressure and temperature.
use of proper gauges and leak-free connections should be ascertained. Burners, hot-plates, flames, or sparking motors should not be in the vicinity. Calculations After correcting for water vapor pressure, the volume of the gas at standard conditions, V STP, is calculated with the use of the ideal gas law. The weight and the density of the gas in the balloon are then calculated with the following equations: Wgas = V STP × d air – (Wballoon + Wstring ) dgas = Wgas /V STP where Wgas is the weight of gas in the balloon, dair is the density of air at standard conditions (1.205 g/L), Wballoon is the weight of balloon, and Wstring is the weight of string. The molecular weight of the gas, MWgas, is then obtained by multiplying the molar volume of 22.400 L/mol by the density of the gas, dgas : MWgas = 22.400 L/mol × dgas g/L This experiment is applicable to gases that are lighter than air, 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. This is to be expected, since the gas volume measurement is good only to 1–3%. In conclusion, this is a simple, easy-to-perform, and low-cost experiment that can easily be completed within one 3-hour laboratory period and is suitable for use in a firstyear introductory chemistry laboratory. W
Supplemental Material
Notes for the instructor and a handout for students are available in this issue of JCE Online.
Hazards Hydrogen, methane, and natural gas are flammable compressed gases. When in use, the gas cylinders should be kept in a fume hood and should be securely anchored. The
Literature Cited 1. Kalbus, Lee; Petrucci, Ralph H. J. Chem. Educ. 1971, 48, 107.
JChemEd.chem.wisc.edu • Vol. 79 No. 4 April 2002 • Journal of Chemical Education
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