The Molecular Weight Of Carbon Dioxide Worksheet

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Chemistry 11
Santa Monica College
The Molecular Weight of Carbon Dioxide
Objectives
The purpose of this experiment is to determine the molecular weight (molar mass) of carbon
dioxide based on measurements of the pressure, temperature, volume and mass of a sample of
the gas. Once obtained, the experimental molecular weight will be compared to the theoretical
molecular weight of carbon dioxide.
Background
From the ideal gas law, PV=nRT
,
it is easy to show that the density of an ideal gas is:
PM
m
=
=
W
D
(1)
where M
is the molecular weight (molar mass) of the gas.
w
gas
V
RT
This can be solved to obtain:
mRT
=
(2)
M
W
PV
Thus, the molecular weight (or molar mass) of a gas can be determined by measuring the
temperature, pressure, volume and the mass of a sample of the gas.
In this experiment the molecular weight of carbon dioxide will be determined. The carbon
dioxide gas is produced by the reaction of calcium carbonate with hydrochloric acid.
CaCO
(s) + 2 HCl (aq)
CO
(g) + CaCl
(aq) + H
O (l)
3
2
2
2
The carbon dioxide gas will be collected in an Erlenmeyer flask that is covered with aluminum
foil. The foil and the fact that carbon dioxide gas is denser than air inhibit the rate at which the
carbon dioxide mixes with the outside air.
The temperature of the gas will be measured by briefly placing a thermometer underneath the
foil.
Since the flask is not sealed the pressure of the gas in the flask will be equal to the pressure in
the laboratory. The laboratory pressure will be measured with a mercury barometer.
The volume of the gas is equal to the volume of the flask. The volume of the flask will be
measured by filling it to the brim with deionized water. From the measured mass of the water
and the known density of water the volume of the water, and thus the volume of the flask, will be
determined. A table of densities of water at various temperatures is given below.
Molecular Weight of a Gas
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