Experiment 3: Determination of Molar Mass by Vapor Density
Background: The Ideal Gas Law can be used to determine the approximate* molecular mass
of a gas through the equation MM PV
gRT
:
::
=. In this experiment, a small amount of a volatile
liquid is placed into a flask (of which you know the volume) and allowed to vaporize. A small
pinhole in the system allows the pressure of the gas to equalize with the air pressure. The tem-
perature of the vapor will be assumed to be the temperature of the boiling water in which the
flask is immersed. At this point, you know g, R, T, V, and P. You will then be able to calculate
the apparent MM.
Scenario: A student obtained a sample of an unknown volatile liquid and placed it into a
0.264 L flask. The student covered the flask with a piece of aluminum foil which contained a
very small pin hole. The student then immersed the flask into a boiling water bath, which she
measured as 100.5 °C** and allowed the liquid to vaporize, forcing out the air in the flask.
(See Figure 1 for set-up.)
Figure 1*The gas or vapor may deviate significantly from ideality since it is not that far above the temperature of its boiling point.
**The temperature of boiling water at 100.5°C at 750. torr may indicate that the water that was used in the boiling water bath was not dis-
tilled water but rather tap water and approached 1m in ionic concentration.When there was no more liquid visible in the flask, the student then placed the flask into cold
water so that the vapor could condense. She dried the flask thoroughly and then reweighed the
flask and determined the mass of the condensed vapor to be 0.430 g. The student then checked
the barometric pressure of the room and found it to be 750. torr.
thermometerLaboratory Experiments