edited by ROBERT REEVES Marlborough School 250 S. Rossmore Avenue Los Angeles. CA 90004
Using a Dumas Bulb To Determine the Molecular Weight of a Volatile Liquid Christopher J. Kawa Illinois Mathematics and Science Academy, 1500 West Sullivan Road, Aurora, IL 60506 A common experiment to determine the molecular weight of a volatile liquid uses a round-bottom flask covered with aluminum foil that has a pinhole in it. Results can be improved by using a vapor density bulb or Dumas bulb.' To use the Dumas bulb successfully for this purpose, its volume must be determined. The most direct method involves filling with water and weighing the water, and calculating the volume based on the density of water. However, the opening of the bulb is so small that filling is difficult. It can be filled by introducing water in the same way the volatile liquid is introduced; that is, heatbig the bulb in the boiling water bath, quickly removing and placing the spout in water, and as it cools a partial vacuum causes water to be drawn inside. This procedure must be repeated several times, and then, to fill the long spout, a hypodermic syringe is needed. Emptying the bulb usually involves shaking the water out, and this is an even longer process, unless interrupted by misfortune. When posed with the problem of designing a more effective method, my student, Mehmet Guler, proposed this alternative method. It uses a simple apparatus consisting of two hollow glass bends, one long, one short, inserted into a twohole #6 stopper. The stopper assembly is placed in a 250mL side-arm Erlenmeyer flask connected to an aspirator as shown in Figure 1. The flask is filled with distilled water. A short piece of rubber tubing connects the long piece of glass tubing to the spout of the Dumas bulb. With the aspirator in operation, one places his finger over the short piece of glass tubing sweeping the air out of the bulb as is evidenced by bubbles appearing in the flask; then, when the finger is removed, air forces the water into the bulb. In rapid fashion, the process is repeated until the entire bulb is filled. To empty, a small amount of water must be first shaken out of the bulb to allow some air in, then reversing the hookup to the glass tubes in the flask holding the bulb with the spout side down (see Fig. 2). Again placing the finger over the open glass tube and alternately removing it allows more air inside the bulb. The entire process of filling and emptying can be done in about 10 min. Results are low by about 5%due to the high vapor pressure of many of the liquids, particularly when warm. This can he minimized by closing off the opening of the bulb with one's finger or a suitable stopper as it cools down to room temperature.
Figure 1.
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A Dumas bulb like that described in this article is available from most scientific supply companies like Sargent-Welch (P. 0. Box 1026, 7300 N. Linder Avenue, Skokie, IL 70077) for $9.00 each.
336
Journal of Chemical Education
Filling the Dumas bulb.
Figure 2.
Emptying the Dumas bulb.
