E10A-1
Experiment 10A
FV 2-4-14
MOLAR MASS OF A LIQUID FROM
THE DENSITY OF ITS VAPOR
MATERIALS: Dry 250 mL Erlenmeyer flask, piece of foil (~3” x 3”), rubber band, 800 mL beaker, 500 mL
graduated cylinder, iron ring, ring stand, wire gauze, Bunsen burner, test tube clamp, Hot Hands.
PURPOSE: The purpose of this experiment is to determine the molar mass of an unknown liquid by measuring
properties of its vapor phase after heating.
LEARNING OBJECTIVES: By the end of this experiment, the student should be able to demonstrate the
following proficiencies:
1. Use the Dumas method to determine the molar mass of a liquid.
2. Given the properties of a gas, calculate its molar mass by using the Ideal Gas Law.
3. Suggest reasons why this type of determination may not give an exact value for the molar mass.
DISCUSSION: In this experiment, the Dumas method will be used to determine the molar mass of a volatile liquid. In
this method, the liquid is heated to completely convert it to its vapor (filling the flask). The properties of
the gas are measured. The molar mass is determined by applying the Ideal Gas Law, PV = nRT, where
P is the pressure (in atm), V is the volume (in L), n is the number of moles of gas, R is the universal gas
constant (0.08206 Latm/molK), and T is the temperature (in K).
PROCEDURE: (work with a partner)
(CAUTION! Some of the unknowns are highly flammable!)
1. Fill an 800 mL beaker with 300 mL of distilled water and place it on a wire gauze on
an iron ring attached to a ring stand. Light your Bunsen burner and adjust the flame.
Carefully raise or lower the beaker on the ring stand so that the flame will be at the
proper height for heating the beaker (don’t burn yourself). Turn off the burner for
now.
2. Obtain a dry 250 mL Erlenmeyer flask from the front of the room. Weigh the dry
flask (no cap) on a top-loading balance to the nearest 0.01 g. Record this value in
the Data Section.
3. Obtain a sample of an unknown liquid from your instructor and pour the liquid into
the flask. Record the unknown number in the Data Section. Return the vial to
your instructor without washing it.
4. Place a piece of foil over the opening of the Erlenmeyer flask and crimp the edges of
the foil tightly around the opening of the flask. Place a rubber band over the foil to
secure it to the flask. Poke 3-4 pinholes into the foil to let the excess gas escape
(you can use a pencil or pen but make sure the holes are small).
5. Carefully attach a clamp to the Erlenmeyer flask and force the flask down into the
water in the beaker until the clamp is at the lip of the beaker. Secure the clamp to
Setup
Flask with foil
