LAB – MOLAR MASS OF A VOLATILE LIQUID
The molar mass is defined as the mass in grams of one mole (6.02 x 10 23 particles) of that
substance. For example carbon dioxide (CO2) has a molar mass of 44.0 g/mol. Because this value is a
ratio, it is an intensive property, meaning its value will not change with the size of the sample. As a result,
intensive properties can be used to identify unknown substances. In this lab, you will find the molar mass
of an unknown volatile (a liquid that evaporates very easily) liquid and use that value to predict the formula
of the unknown. This will be done by vaporizing a sample of the volatile liquid, and then using the ideal
gas law to calculate the number of moles of vapor produced. Modern analytical labs have automated
equipment that can be used to identify unknowns, but this technique will still be useful in illustrating the
concept.
Equipment, 125 mL Erlenmeyer flask, 10 mL graduated cylinder, 600 mL beaker, aluminum foil, rubber
band, pin, ring stand, utility clamp, hot plate, thermometer, barometer, analytical balance,
100 mL graduated cylinder
Materials: unknown volatile liquid, water
Procedure:
1) Place the Erlenmeyer flask, foil, and rubber band on the balance and record the mass.
2) Measure 3 mL of the unknown liquid and add it to the flask.
3) Use the foil and rubber band to tightly seal the opening of the flask. Use the pin to make as small a hole
as possible in the center of the foil.
4) Place the 600 mL beaker on the hot plate. Clamp the flask so it is completely inside the 600 mL beaker
and tilt the flask slightly. Attach the clamp to the ring stand.
5) Add enough water to the beaker to completely surround the flask.
6) Slowly heat the water and observe the vaporization of the unknown liquid.
7) When the unknown liquid has completely evaporated, remove the flask from the water and measure
the water’s temperature. Record this value in your data table.
8) Record the barometric pressure. Make sure it is in kPa or atm.
9) When the flask has cooled back to room temperature, dry the outside and record the mass of the flask,
foil, and rubber band.
10) Remove the foil, fill the flask with water, and use a graduated cylinder to measure the volume of the
flask. Record the volume.
Data:
(a) Mass of flask, foil, and rubber band
(b) Water Temperature
(c) Barometric Pressure
(d) Mass of flask, foil, rubber band, and liquid
(e) Volume of the flask
Calculations:
1) Use the ideal gas law (PV=nRT) to find the number of moles (n) of vapor in the flask.
0.0821
L  atm
L  kPa
 8.31
mol  K
mol  K
LAB – MOLAR MASS OF A VOLATILE LIQUID
2) Calculate the mass of the vapor. (d – a)
3) Calculate the molar mass of the unknown (#2 / #1)
4) Calculate the percent error of your value. Your instructor will give you the molecular formula
of your unknown.
Conclusions:
1) Describe any sources of error in your experiment.
2) How could your water temperature (b) be used to identify your unknown substance?