A Simple Molecular Weight Experiment Most freshman chemistry courses include an experiment to determine the molecular weight of a gas or volatile liquid. The method most commonly employed is the Dumas method. Since an experiment of this type is likely to be performed very early in the freshman course, we have sought s. method that is conceptually simpler than the Dumas method and easier to perform. Our students have had considerable success in determining the molecular weight of CO2by the following method. A rubber hdllloon is weighed to the nearest milligram. One or two pieces of Dry Ice (-lg), freed of ice condensate, are introduced into the balloon; the balloon is tied offsecurely a t the neck; and the balloon is reweighed. The balloon is then quickly inserted into a 1-1 graduated cylinder which is filled with water and inverted in a sink full of water. After the Dry Ice has completely sublimed (ahout 15 min), the belloon is punctured with a s t 8 metal wire inserted through the mouth of the cylinder. The water levels are equalized and the volume of GOnrecorded. Typical student data. yields. volume of about 600 ml for a 1.00-g sample. Depending upon the degree of sophistication desired, the data may be handled in a variety of ways. The simplest is to use a molar volume of 22.4 1 with no correction for nonstandard temperature and pressure. The student determines, by propoition, what weight sample would he required to produce 22.4 1of gas. The result is a molecular weight value of about 36-39. However, the gas corrections are simple enough to make, either by cdculstion or by providing s. value of the molar volume for the given temperature and barometric pressure. With these corrections typical results are in the range 4246. A still further refinement makes allowance for water vapor in the balloon derived from possible moisture condensate on the Dry.1~~. The exact water vapor pressure cannot he determined, hut the equilibrium vapor pressure can be assumed as an upper limit. If an experiment is done with care snd the several m e c t i o n s made, a value is obtained that is very close to 44. Among the sources of error in the experiment are the introduction of water vapor from moisture condensate on the Dry Ice, slight leakage of gas from the balloon during weighing, and the solubility of C02in water. None of these proves to be particularly significant, since the volume measurement'itself is good only to 1-2%. For example, in an experiment designed to determine the rate of dissalving of C02 in water, the diminution of gas volume after rupture of a balloon amounted only to about 1% in 15 min. LEE KALEUS
RALPHH. PETRUCCI CALIPORNIA STATECOLLEGE SANBERNARDINO
Volume 48, Number 2, February 1971
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