Molecular weight determination by an improved temperature monitored vapor density method

Abstract

These authors present an article that provides an excellent way to attain accurate molecular weights for volatile materials.

A Closer Look at the Common Molar Mass Experiment

“…At exactly that point, the flask should be removed from the boiling water bath and “not a second later”. Different techniques have been reported to precisely determine the moment of complete evaporation. , However, it should be apparent from the theoretical considerations that in principle a decent accurate molar mass cannot be determined with the “classic” method, unless the experiment is done in a way that compensates for the mass of the replaced air. The “good” achievable results can be explained as follows: If the flask is removed from the boiling water when still a small quantity of liquid is left and the mass of that liquid approaches the mass of the displaced air for the RT flask conditions, then a “better” result would be expected.…”

“…The probably most common, modified version of the Dumas bulb method applied for the “classic” Molar Mass experiment is the “aluminum foil over an Erlenmeyer flask” approach . Due to its simplicity it is very attractive, widely used, and has been implemented by many schools. − At New Mexico Highlands University (NMHU) this experiment is also taught in the General Chemistry laboratory. The experiment can be found in LibreText Chemistry under “11B: The Dumas Method (Experiment)” (Santa Monica College) .…”

Some Insights Regarding a Popular Introductory Gas Law Experiment

“…If the flask is sealed after the excess vapor has escaped and prior to condensation, the problem described in this report disappears, since air can no longer return to the flask once the vapor is condensed (1). Other ways around the problem described in this paper include drawing a sample gas into a vacuum so that air is never present (9), and maintaining a constant amount of air in the apparatus at constant pressure by allowing the volume to change (10).…”

“…Determining the molar mass of gases or easily vaporized liquids is described in virtually all first-year college chemistry texts to a greater or lesser extent, and is an appropriate activity to engage students once they have been exposed to the gas laws. The Dumas method provides a relatively simple way to determine the molar mass of volatile chemical compounds, and has been described in various places (1)(2)(3)(4)(5)(6). This report alerts readers to a potential source of error in one approach to the Dumas method as it is often practiced in introductory chemistry laboratories.…”