Chem 112-2011 Determining the Molar Mass of a Gas Using the Dumas Method
Introduction
Compounds that are liquids at room temperature but that have relatively low boiling points can be used
in a clever experiment for determining molar mass. The idea rests on the ideal gas law, solved for moles
of gas:
n
PV
RT
n = moles of gas
P = pressure in atmospheres; 1 atm = 760 mm Hg
V = volume in liters
T is temperature in Kelvin (K = oC + 273.15)
R = gas constant = 0.082057 Latm/Kmol
If you know P, V, T, and R, you can solve for the moles of gas
present, n. Measuring P, V, and T is relatively easy. The trick
is to get a sample of gas that is composed of only the gas
you are measuring, and not any air. The problem is solved
with the apparatus pictured to the right:
Procedure
1. Add about 10 mL of the volatile liquid is placed in a 250 mL Erlenmeyer flask.
2. Add a single boiling stone.
3. Cap the flask with tightly sealed aluminum foil, using a rubber band to seal it.
4. Poke a tiny (tiny!) hole in the aluminum foil. What kind of hole? Tiny.
5. Place the flask in big beaker of boiling water.
At this stage the volatile liquid boils. It produces about 2 liters of vapor. That newly formed gas
"flushes" any air out of the flask. The air is forced out the tiny hole in the aluminum foil. Once all
the liquid has boiled, the flask is filled with just the gas formed from the liquid. Mission
accomplished.
Be sure the flask is as far down in the water bath as possible without getting the aluminum foil wet.
Also, refill the beaker with water between trials if needed to keep the water level high enough.
6. As the liquid vaporizes, you will at first see recondensed liquid streaming down the inside of the glass.
As the flask warms, that will stop. Watch the pool of liquid at the bottom. The vaporization is complete
when the liquid just disappears.
7. When vaporization is complete, remove the flask (which is very hot) by removing the clamp from the
ring stand. Quickly rinse the flask walls in cold water but be careful to not splash water near the top of
the flask. You should see recondensed liquid in the flask at this time. Be sure to dry the outside of the
flask before doing any weighing.
What to measure, how to calculate:
You r goal is to determine the molar mass:
grams unknown
moles unknown
To find moles, use the ideal gas law after finding P, V, and T.
P: read the pressure using the barometer that is in the back of Room 203.
V: the flask says 250 mL, but that's not its volume. Figure out a way to determine its actual volume.
T: what is the temperature of the gas when it is in the gaseous state?
To find grams, you need to figure a way to weigh the compound that fills the flask as a gas. Figure it out.
The problem with this experiment is that if you don't boil long enough, you leave some liquid
unvaporized in the flask and it erroneously adds to the mass measured or you boil too long and some of
the vapor escapes and is replaced by air, leaving you with too small a mass. How do you guard against
these errors: repeat the experiment to make sure you get the same result.
Prelab Assignment:
Name: __________________
Section/Day/Time: ____________________
Question 1.
A gas has a volume of 235 mL, a pressure of 710 mm Hg, a temperature of 55 oC, and a mass of 1.22 g.
What is its molar mass?
Question 2.
How are you going to determine the volume of the Erlenmeyer flask in this experiment?
Question 3.
How are you going to determine the mass of the gas. Be specific about what you will weigh.