DETERMINATION OF MOLAR MASS OF BY VAPOR DENSITY
One of the properties that helps characterize a substance is its molar mass. If the substance in question is a
volatile liquid, a common method to determine its molar mass is to use the ideal gas law, PV = nRT. Because
the liquid is volatile (from Latin volare, “to fly”), it can easily be converted to a gas. While the substance is
in the gas phase, you can measure its volume, pressure, and temperature. You can then use the ideal gas law
to calculate the number of moles of the substance. Finally, you can use the number of moles of the gas to
calculate molar mass.
OBJECTIVES
In this experiment, you will
 Evaporate a sample of a liquid substance and measure certain physical properties of the substance as it
condenses.
 Determine the molar mass of an unknown volatile liquid.
MATERIALS
Thermometer
two 400 mL beakers
ice
tissues or paper towels
ring stand
two utility clamps
aluminum foil
unknown volatile liquid
fume hood
test tube, 13  100 mm,
needle
hot plate
analytical balance
Test tube holder
Figure 1
PROCEDURE
1. Obtain and wear goggles. Conduct this experiment in a fume hood or well-ventilated area.
2. Trim a piece of aluminum foil so that it just covers the top of a small, 13  100 mm, test tube. Use a
needle to make a SMALL hole in the foil. Place a Styrofoam cup on a balance and tare (zero). Measure
and record the mass of the test tube and foil.
3. Prepare a hot-water bath by warming about 300 mL of tap water in a 400 mL beaker. Keep the beaker on
a hot plate once the water is warm.
4. Use a second 400 mL beaker to prepare an ice-water bath.
5. Obtain a liquid sample of an unknown volatile compound. Measure about 0.5 mL of the liquid into the
test tube and quickly cover the test tube with the aluminum foil.
6. Measure and record the mass of the test tube, foil, and liquid sample.
7. Place the test tube in the hot-water bath. Make sure that the foil is above the water level (see Figure 1).
8. Immerse the thermometer in the water bath, making sure to keep it from touching any of the sides of the
glass.
9. Heat the beaker of water to boiling and maintain the boiling as your sample of liquid vaporizes. Note
that some of your sample will escape the test tube through the needle hole in the foil. This process also
serves to flush the air out of the test tube.
10. Keep the test tube in the boiling water bath for at least three minutes after all of the liquid in the test
tube has vaporized. Record the temperature of the boiling-water bath, which will be used in the ideal
gas law calculations.
11. Use a test-tube holder to quickly transfer the test tube to the ice water bath. Cool the test tube for
about one minute, then remove it and dry it completely. Measure the mass of the test tube, the
condensate, and the aluminum foil top.
12. Record the barometric pressure in the room.
13. Rinse out the test tube and fill it to the top with tap water. Cover the test tube with aluminum foil.
Measure and record the mass of the test tube, water, and foil.
DATA
Trial 1
Trial 2
Mass of test tube and foil (g)
Mass of test tube, foil, and unknown liquid (g)
Mass of unknown liquid (g)
Mass of test tube, foil, and condensate (g)
Mass of condensate (g)
Final temperature of water bath (ºC)
Barometric pressure (kPa)
Mass of test tube, foil, and water (g)
Mass of water in test tube (g)
Volume of test tube (mL)
% Error
ANALYSIS (Show work)
1. Determine the mass of the condensate.
2. Use the mass of the water in the test tube and the density of water (1g/mL) to calculate the volume of
the test tube.
3. Use the calculations from Questions 1 and 2 above, along with the temperature of the boiling water
bath and the barometric pressure of the room, to calculate the molar mass of your unknown
compound.
4. Using the mass of the condensate and the calculated molar mass of your unknown, determine the
amount of kinetic energy (in kJ) present in your unknown sample when it was in the vapor phase.
5. Your unknown liquid has the following percent composition: 52.1% carbon, 13.1% hydrogen, and
34.8% oxygen. Determine the empirical formula of your unknown.
6. Using the empirical formula and the molar mass calculated in Question 3, determine the molecular
formula of your unknown.
7. Common names for your unknown liquid are grain alcohol and drinking alcohol, so called because it
has been historically produced by the fermentation of the sugars in grains by yeast to make wines and
beer. The largest use of this alcohol, however, is as a motor fuel and gasoline additive. Draw the
structural formula of your compound and give its IUPAC name.
8. Determine your percent error for the calculation of the molar mass of your unknown.
Discussion & Conclusion
In this section of your lab report, be sure to include possible sources of error. Keep in the mind the following
questions when you write this section:
(a) Was the vapor really “ideal”? Explain.
(b) If all of the vapor had not condensed to a liquid when you cooled the test tube, how would your
calculations have been affected?
(c) If the test tube was not thoroughly wiped dry before massing, how would this affect the molar mass
calculation?
Extension Questions
1. When an unknown liquid was analyzed to determine its molecular mass, using the techniques of this lab
exercise, the following measurements were obtained:
Volume of test tube = 160.6 mL
Temperature of the boiling water = 98.9 oC
Barometric pressure = 754.6 mm Hg
Mass of test tube + condensate = 97.71 g
Mass of test tube = 97.48 g
Calculate the molar mass of the liquid.
2. A gas has a density of 1.25 g/L at STP. What is its molecular mass?
3. The rubbing alcohol that we buy in stores is actually a mixture of water and 2-propanol. Explain how
you would separate the compounds in such a mixture to isolate a pure sample of 2-propanol.