Molecular Mass of a Volatile Liquid
Purpose

To determine the molecular mass of the vapor of a volatile liquid.
Introduction
It is often useful to know the molecular mass of a substance since it is one of the
properties that help characterize the substance. If the substance is a volatile liquid, one
common way of determining its molecular mass involves using the ideal gas law, PV =
nRT. Since the liquid is volatile, it can easily be converted to a gas. While it is in the gas
phase, its volume, temperature and pressure are measured. The ideal gas law will then
allow the calculation of the number of moles of the substance present.
The mass of the gas is found by cooling the gas so that it condenses back into a
liquid and then determining the mass of the condensed liquid. Molecular mass can be
calculated from the number of moles and the mass of the sample.
Procedure
1. Weigh accurately a clean, dry 125-mL Erlenmeyer flask together with a 6 x 6 cm
square piece of aluminum foil and a 15 cm length of thin copper wire.
2. Using a small graduated cylinder, transfer a 2 mL sample of the volatile liquid into
the flask.
3. Place the aluminum foil on the mouth of the flask and carefully press it into place so
that it forms a reasonably tight closure over and around the top of the flask. Take
care not to tear or pierce the foil during this operation. Place the wire around the
top of the flask and twist the ends together so that the foil is held tightly in place.
Place a second piece of foil (8 x 8 cm) over the first one and hold it by means of a
rubber band. With a pin, punch a tiny hole through the pieces of aluminum foil near
the middle of the opening of the flask.
4. Add approximately 200 mL of water and four or five boiling chips to a 600 mL
beaker. Add approximately 2 mL of 3 M HCl to the beaker to help prevent hard
water scale from building up on the outside of the flask. Place the 125 mL flask and
contents in the beaker and clamp it into place so as much of the flask as possible is
submerged in the water. You may have to adjust the amount of water.
5. Heat slowly so that the water is brought to boiling in about 10 minutes. When
boiling commences, lower the temperature and continue to boil very gently until no
trace of liquid is left in the flask. Then boil for two more minutes. Record the
temperature of the boiling water.
6. Turn off the hot plate and immediately remove the flask from the water. Dry the
outside of the flask with a towel and remove the outer foil. Remove any moisture
from the inner foil with a paper towel. If the experiment has been successful, a
small amount of liquid should condense inside the flask as it cools.
7. Set the flask, with inner foil and wire intact, on a clean, dry surface to cool. When
cooled to room temperature, weigh the covered flask and contents.
Molecular Mass of a Volatile Liquid
8. Determine the volume of the flask by filling it to the lip with distilled water and
measuring the mass, along with the inner foil and wire. Measure the temperature of
the distilled water.
Calculations
1. Calculate the volume of the flask, using the mass and density of the distilled water.
2. Calculate the moles of gas in the flask, using the ideal gas equation.
3. Calculate the molecular mass of the volatile liquid, using the mass and moles of the
liquid.
Data
Mass of flask, foil & wire
Temperature of boiling water
Mass of flask, foil, wire &
liquid
Mass of liquid in flask
Mass of flask, foil, wire & water
Mass of water in flask
Temperature of distilled water
Density of distilled water
Barometric pressure
Results
Volume of flask
Moles of gas
Molecular mass of liquid
Molecular Mass of a Volatile Liquid
Questions
1. You continued to heat the sample in the boiling water for two minutes after the
liquid had vaporized. Why doesn’t the vapor escape during that time?
2. Determine whether each of the following errors would make the final calculated
value of the molecular mass too high, too low or would cause no change. Explain
your reasoning.
a. All of the liquid did not vaporize.
b. The temperature of the boiling water was 2.0C above the temperature recorded.
c. A small amount of scale from tap water was deposited on the outside of the flask.
d. The flask was not completely filled with water in the last part.