Determination of Molecular Weight by Gas Density Measurements

One of the simplest methods for determining molecular weights is based on the ideal gas
PV nRT =
where P is the pressure in atmospheres, V is the volume in liters, n is the number of moles, T is
the temperature in K of a gas sample, and R is the gas constant, which is equal to
0.0821 L atm/K mol. If P, V, and T for a gaseous sample can be measured, the gas law can be
used to calculate the number of moles of gas present. If, in addition, the weight m of the sample
can be determined, its molecular weight can be calculated, since
PV =
Dumas developed a simple procedure in which the pressure, volume, temperature, and
mass of a gaseous sample could be easily measured, and its molecular weight calculated. In the
procedure, an excess of a volatile liquid is placed in a flask of known mass and heated at
constant temperature, in a water bath, until all of the liquid vaporizes. This drives both the air
and any excess vapor out of the flask. The flask then contains vapor at a known T (temperature
of the water bath), P (atmospheric pressure), and V (volume of the flask). To determine the mass
of the vapor, the flask is cooled and reweighed. The increase in mass equals the mass of the
The Dumas method is used in this experiment to determine the molecular weight of an
unknown volatile liquid.
RELEVANT PROBLEMS FROM THE TEXT (Chang, 10e): Example 5.9, Problems 5.43,
5.44, 5.47
1. Obtain a thermometer, flask, 1-hole stopper (or piece of foil), and supporting wire from
the stockroom, and an unknown from the instructor. Record the unknown number on the data
2. Put about 250 mL of tap water in a 400-mL beaker, and heat the water to boiling on a
hot plate.
3. While waiting for the water to boil, weigh the flask and 1-hole stopper (or foil) on the
analytical balance to ±0.0001 g. Record the mass on the data sheet.
4. Pour about one third of the unknown into the flask. Replace the 1-hole stopper.
Immerse the flask in the hot water bath and support it with the wire, cork, and clamp as
demonstrated by the instructor. The flask should be immersed in the hot water up to the neck
of the flask to prevent condensation of the liquid. Replace the water in the beaker as it
evaporates. Measure and record the temperature of the boiling water.
5. When all the liquid has been vaporized, remove the flask from the water bath. Allow
the flask to cool to room temperature.
6. When cool, dry off the outside of the flask.
7. Weigh the flask, 1-hole stopper, and condensed vapor to ±0.0001 g on the analytical
balance. Record the mass on the data sheet.
8. Repeat the experiment by adding half the remaining unknown to the flask, heating in
the water bath, cooling, and reweighing. Repeat a third time with the remaining unknown.
9. The total volume of the flask is 50.0 mL. Record this volume. Record the atmospheric
pressure from the barometer in the lab (or your instructor may give you the value to use).
10. Using the ideal gas law, calculate the molecular weight of the liquid.
11. Clean the flask by rinsing it thoroughly with acetone.



































Sample Solution