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Dr. Buckley
e-mail: gbuckley@cameron.edu
Experiment #15 – Determining the Molecular Weight of a
Volatile Liquid by the Dumas Method
Laboratory Overview
CHEM 1361
August 2010
Gary S. Buckley, Ph.D.
Department of Physical Sciences
Cameron University
Table of Contents
(you may click on any of the topics below to go directly to that topic)
•Experimental Objectives
•Ideal Gas Law
•Experimental Procedure
Experimental Objectives
•Acquire experimental data related to the ideal gas law and solve for
missing quantities
•Become acquainted with the Dumas method for determining the molar
mass of a volatile liquid
•Find the molar mass of a volatile substance from its volume,
temperature, and pressure using the ideal gas law
Ideal Gas Law
The ideal gas law expresses the mathematical relationship between the
pressure, volume, temperature, and number moles of an ideal gas. It may be
given as:
PV = nRT
where P is the pressure, V the volume, n the number of moles of gas, R a
constant, and T is the temperature. The units chosen for P and V must be
consistent with the value used for R, the ideal gas constant.
R has only a single value, but can be expressed in different sets of units (similar
to length, for example, 1 yard = 3 feet = 36 inches, all of which represent the
same length). A couple of typical methods for expressing R include:
R 0.08206
L atm
L torr
62.36
mol K
mol K
In applying the ideal gas equation, the units chosen for pressure, volume , and
temperature must match those of the selected gas constant.
More on Application of the Ideal Gas Law
In this particular experiment, you will take measurements of the mass of the
vapor of a sample, its pressure, volume, and temperature in order to determine
the number of moles in a flask. Based on the mass and calculated value of n, the
number of moles, the molar mass can be determined.
From the ideal gas law
n
PV
RT
so experimental determination of P, V, and T will give the number of moles of gas
in a container.
By measuring the mass of the gas the molar mass may be determined from
molar mass =
mass of substance
mol of substance
Experimental Procedure
In the Dumas method, a volatile liquid is placed in a dry flask.
A piece of aluminum foil with a pinhole is placed over the opening
in the flask and the apparatus placed in boiling water. The liquid
will vaporize slowly through the pinhole until it is gone from the
flask. At the point at which the last bit of liquid disappears, the
flask is filled only with the vapor of the volatile liquid. Once the
flask is removed from the boiling water, the will condense and its
mass may be determined.
Al foil cap with pin hole
The temperature is assumed to be that of boiling water, the volume
of the flask is determined by measuring the mass of water required
to completely fill the flask, and the pressure is atmospheric pressure
determined from a barometer in the room. From application of the
ideal gas law the number of moles of condensate – effectively the mass of vapor in
the flask - may be determined.
The trickiest part of this experiment is determining when the last drop of liquid has
vaporized. If you quit boiling too soon you will have liquid in the flask that will be
counted as condensate; if too late you will lose some of the condensate through
vaporization.
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