Name __________________________________________________ Date ________________________
NORTH ALLEGHENY SENIOR HIGH SCHOOL
AP Chemistry
MOLAR MASS OF A VOLATILE LIQUID
INTRODUCTION:
If a substance behaves as an ideal gas, we can easily calculate its molar mass if we measure the mass m of a
volume V of the pure gas at known temperature T and pressure P. The ideal gas law is PV = nRT, where n =
moles of gas. But n = m/Mw where Mw equals molar mass. Therefore, Mw = mRT/PV. This method is
applicable to any substance, provided the chemist has sufficient skill and sophisticated gas-handling equipment to
obtain the sample under controlled conditions. However, if the sample is a liquid at room temperature, with only a
modest vapor pressure, and if it can be completely vaporized at some readily controlled high temperature (such as
that of boiling water), the technique can be simplified; it is known as the Dumas method, especially among organic
chemists. This experiment is not different from the Dumas method, except that the equipment is extremely simple
and inexpensive.
An arbitrary amount of sample is put into the vessel of known volume (an Erlenmeyer flask) and vaporized
at approximately 100oC, the excess escaping through a pinhole opening in the flask cover. The mass of the vapor is
obtained after the flask has been cooled and the vapor has condensed. We assume that the pressure of the vapor in
the hot flask is the same as that in the laboratory as shown in the barometer. There is a basic flaw in the method.
We obtain the weight of vapor by subtracting the mass of the empty vessel from the final mass, on the assumption
that the air content of the flask is the same during each weighing, which would be true only if the sample's vapor
pressure were negligible at room temperature. In a typical case, neglect of this error gives results about 5% low.
Usually a Dumas molar mass was used to determine a molecular formula from an empirical formula and so this
small error would not affect the decision.
Each student will determine the molar mass of an unknown liquid assigned by the instructor. At least two
complete determinations should be run. If the agreement is poor (worse than 5%), try a third, if time allows. If you
are required to make a correction for vapor pressure, your instructor will provide the value of the room temperature
vapor pressure of the liquid. When the lab is complete the value of the molar mass of the volatile liquid should be
known.
PROCEDURE:
1. Construct a hot water bath similar to the setup on the next page. Allow the bath to heat as you move on to
the rest of the lab.
2. Clean and flame dry a 125 mL Erlenmeyer flask. Allow to cool.
3. Cut a square of aluminum large enough to cover the mouth of the flask. Don't make the aluminum square
too large. Just enough to cover the mouth.
4. Measure the mass of the flask and the aluminum cover on the milligram balance.
5. Put one full pipet worth of your unknown liquid into the flask (use the pipets provided).
6. Press the aluminum cover over the mouth of the flask, and punch a pinhole in the cover using a tack.
7. Mount the flask in a beaker of water so that only the top of the neck protrudes (see figure 1).
Figure 1: Simplified apparatus
for the Dumas method
8. Heat the flask until you feel all of the liquid has evaporated. At that point continue to heat for another
minute at a gentle boil so that the vapor completely fills the flask. (the water in the beaker must be gently
boiling for at least 2-3 minutes before removing)
9. Remove the flask from the water bath and without any delay immerse the flask in a beaker of cold water.
10. Carefully wipe the water from the outside of the flask. Delicately try to lift the aluminum from the sides of
the flask without removing the aluminum to dry any water that is under the aluminum. Water tends to
collect in the folds of the foil around the neck of the flask. Be very diligent about making sure the flask and
aluminum is dry. Once the flask and aluminum is dry, obtain a final mass (this should all be done quickly).
11. Measure the volume of the flask. You can be creative in this step.
12. Repeat steps 1-10 using a clean, dry flask and a new piece of foil.
13. Obtain the barometric pressure (your instructor will provide this).
CALCULATIONS:
1. Using the data collected from above and your knowledge of the ideal gas equation determine the molecular
weight of the unknown liquid. Ignore the vapor pressure of the unknown liquid in this step.
2. The unknown you are using has a vapor pressure of 75 mm Hg at 20oC. Think about the following
discussion concerning the experiment:
 The total pressure in the experiment is the same for both the initial and final weighing, but in the
final weighing, the air content is 75 mm Hg less than the initial. Calculate the mass of the excluded air
from n = PV/RT and using 29.0 g for the molecular mass of air.
 If the unknown were completely nonvolatile and no air was excluded, the final weighing would have
included the mass of the air calculated above. Adding the excluded air correction to your mass of volatile
liquid collected in the data table, recalculate the molar mass for the volatile liquid.
QUESTIONS:
1. Determine the average of your two MW determinations (after correction). If it is known that the MW of
your unknown is 88.1 g/mol, determine your percent error.
2. Was the vapor you investigated really "ideal" in the experiment (was it at a temperature that put it far above
its boiling point)? Explain the difference between an ideal and non-ideal gas.
3. When placed in the ice bath, did all of the vapor within the flask condense into a liquid? Explain your
answer with respect to the concept of vapor pressure.
4. Why is it not necessary to be precise when the liquid is measured out into the flask?
5. Hypothetically, imagine the following mistakes were made when carrying out the experiment. What effect
does each have on the calculated molecular mass? Be specific. For example, too large because. . .
a. Only part of the flask was immersed in the boiling water bath (think about the neck of the flask
sticking out), so the temperature in part of the flask was less than that of the water bath.
b. Three pipets of the liquid were initially placed in the Erlenmeyer flask instead of the recommended
2 pipets.
c. The mass of the condensed liquid was not determined quickly. Instead, the flask was allowed to
stand for a while before its mass was measured.
6. Suggest how you could perform the experiment on an unknown liquid that boiled at 120 oC, keeping the
setup of the experiment essentially intact.
CONCLUSION: Summarize via a paragraph.