L> Molar Mass of a Volatile Liquid Molar Mass of a Volatile Liquid by the Dumas MethodObjectivesBackground: ideal gas lawProcedure OverviewObjectivesDetermine the molar mass of a volatile liquid.Use the ideal gas law in connection with an experiment.Background: ideal gas lawA volatile liquid (or solid, for that matter) is one which evaporates easily.
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It is not crucial to get exactly 5 mL.Heat to vaporize liquidWe will use Bunsen burner flames to heat the water baths. Be careful not to let your flames get too high, because our volatile liquids are flammable.Diagram of apparatus for our experiment.Photo of apparatus of experiment as done at Rutgers University.Diagram of apparatus as Dumas described it.Put a boiling stone in the water bath too (but you don”t have to know its mass).Keep the flask slightly tilted; it will be easier to notice when the liquid disappears.Keep as much of the flask under water as possible.Measure water bath temperature and atmospheric pressureHeat the water bath gently. The assumption that the water bath and the vaporized liquid have the same temperature is only reasonable if the water is being heated gently (with the temperature not rising rapidly). Don”t let the water boil.Measure the temperature quickly once the liquid vaporizes, but keep the wires of the thermal probe away from the flame and iron ring.You may measure the atmospheric pressure any time during the lab period. Your instructor will show you how to use the barometer in the lab.Condense vapor in flaskRun cold water over flask.Determine mass of flask, boiling stone, foil cap, and condensed vaporThere will be less volatile liquid left than there was initially; that is what we expect. You put in more than enough liquid so its vapor would fill the flask and the excess left through the pinhole. The condensed vapor (i.e., the remaining liquid) has the same mass as the vapor which exactly filled the flask.Be sure the foil cap is not wet from boiling water or condensed water vapor. Dry the neck of the flask, if necessary, keeping the foil cover on.Determine volume of flaskThe flasks are nominally 250 mL, but 250 mL is not an exact measurement; furthermore, it refers to the volume of liquid it holds in normal use, so it excludes the volume of the flask”s neck.After you are finished with all determinations, rinse the flask with acetone, and then with water, to remove all remnants of the oily volatile liquids.Now fill your flask (to overflowing) with tap water, and measure with a 500-mL graduated cylinder the volume it contains.