CM1131_practical_5 - CM1131 Name Lab Group B1 Date 7-Mar-07...

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CM1131 Name: ________ Date: 7-Mar-07 Lab Group: B1 LAB REPORT E XPERIMENT 5 DETERMINATION OF MOLECULAR MASS BY DUMAS’ METHOD A IM To determine the molar mass of an unknown volatile liquid by measuring its vapor density. I NTRODUCTION The Dumas method of determining molar mass assumes the ideal gas law, which holds for vapors at atmospheric pressure and approximately 10 degrees above the vapor’s boiling point. Pressure should be much lower than critical pressure and temperature much higher than critical temperature for the ideal gas law to be assumed. Since liquids are more easily handled than gases, the source of the vapor is usually a volatile liquid which is heated until it vaporizes. In the Dumas method, the volatile sample in the bulb is heated in a water bath. The evaporated vapor will push the air out of the bulb, and then escape out of the bulb until pressure in the bulb equals atmospheric pressure. The bulb is then sealed and, dried, cooled and reweighed. The difference between the initial weighing of the empty bulb and the sealed bulb is the difference in mass between the air occupying the volume of the bulb at room temperature and the sample vapor occupying the volume of the bulb at the water bath temperature. Knowing all relevant parameters such as temperature, pressure, volume and composition of air, and applying the ideal gas law, the sample’s mass and density can be easily determined.
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