MOLAR MASS OF CARBON DIOXIDE
Introduction:
The Ideal Gas Equation (PV = nRT) may be used to determine the molar mass of
a gas or vapor. In order to do this, the pressure, volume, mass and temperature of the gas
must be known. These are all quantities that can be easily measured in the laboratory.
The equation is used to solve for the moles of the gas. From the number of moles and the
mass of the vapor, the molar mass is determined.
In this experiment, a PETE beverage bottle will be used as the sample container.
The cap of the bottle has been fitted with a tire stem so that the pressure of the gas inside
can be measured with a pressure gauge. The volume of the bottle will be used as the
volume of the gas. The temperature of the water bath will be used as the temperature of
the sample.
The gas that will be used in this experiment is carbon dioxide. A small amount of
dry ice will be placed in the bottle and allowed to sublime. When nearly all of the dry ice
has sublimed, the cap will be placed on the bottle and the bottle will be placed in a water
bath. After a few minutes it is assumed that the bottle, gas, and water bath are all at the
same temperature. It is at this time that the measurements should be made.
Purpose:
The purpose of this experiment is to determine the molar mass of carbon dioxide.
Equipment/Materials:
PETE beverage bottle
cap (fitted with valve stem)
large graduated cylinder
balance
dry ice
thermometer
bucket for water bath
pressure gauge
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Safety:

An apron and goggles should be worn in the lab.

Do not put the cap on the bottle until nearly all the dry ice has sublimed.

Do not handle the dry ice. Frostbite can result.
Procedure:
1. Obtain a beverage bottle that has been fitted with a tire stem. Make sure that
the interior and exterior are dry. Weigh the bottle and record the mass in the
data table.
2. Using 1.29 g/L as the density of air, calculate the mass of air in the “empty”
bottle. Subtract this from the value obtained in Step 1 to obtain the mass of
the “empty” bottle. Record these values in the data table.
3. Place a sample of dry ice in the soft drink bottle. When nearly all of the dry
ice has sublimed, put the cap on the bottle. Use care in handling the dry ice –
it can cause frostbite.
4. Place the bottle in a water bath for several minutes. It may be necessary to
use a ring and ring stand to hold the bottle under the surface of the water.
Before removing the bottle, measure the pressure of the gas with the pressure
gauge. This value will later be converted to atm of pressure. Record the
temperature of the water bath when the bottle is removed. Record the value in
the data table.
5. Atmospheric pressure in the English system is often measured in psi (pounds
per square inch). Standard pressure is 14.7 psi. The type of pressure gauges
we are using measure pressures above atmospheric pressure. To find the
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pressure of the gas in the bottle, standard pressure must be added to the
reading on the pressure gauge. Record these values in the data table, and
convert the pressure of the gas to atm.
6. Dry the outside of the bottle and weigh. Subtract the mass of the “empty”
bottle to determine the mass of the carbon dioxide.
7. Fill the bottle with water to obtain the volume of the bottle. This
measurement is made at this time so that the bottle is dry and free of excess
water vapor when the dry ice is added.
8. Substitute the appropriate values into the PV = nRT equation, and solve for
moles. Using the number of moles and the mass of the gas, determine the
molar mass of the gas.
9. Using the same cap, use another bottle to repeat the procedure. Do not
assume that the masses of two different bottles will be the same.
10. Calculate the percent error for your determinations.
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MOLAR MASS OF CARBON DIOXIDE
Data:
Trial 1
Trial 2
Volume of bottle (CO2)
Mass of bottle + air
Mass of air
Mass of bottle
Pressure lb/in2
Pressure CO2, lb/in2
Pressure CO2, atm
Temperature
Mass of bottle + CO2
Mass CO2
Calculations:
1. Calculate the number of moles of carbon dioxide for each trial.
2. Using the mass and the number of moles of carbon dioxide, calculate the molar mass
of carbon dioxide.
3. Calculate the percent error for each trial.
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