ACADEMIC CHEMISTRY
NAME:
DETERMINING the MOLAR MASS of BUTANE USING the IDEAL GAS LAW
INTRODUCTION
The ideal gas law states that the product of the pressure and volume of a gas is
qual to the product of the number of moles of the gas, its Kelvin temperature and a
constant. Which simplifies to: PV = nRT in equation form. R is the universal gas
constant and its values are 0.0821 L atm/ mol K or 62.4 L mmHg/ mol K depending on
which units of pressure are used during the experiment.
In this lab, we will determine the mass, pressure, volume and temperature of a
sample of butane gas that has been released from a lighter and collected through water
displacement. The mass of gas will be determined by weighing the lighter before and
after the gas is released. The volume of gas will be measured using water
displacement. The barometric pressure will be found and water vapor pressure will be
subtracted to determine the pressure of the dry butane using Dalton’s law. Using this
data, it will be possible to determine the experimental molar mass of butane, C4H10.
In order to calculate molar mass (MM) using the ideal gas law, some
substitutions must be made.
n=
grams
MM
Substituting into the ideal gas law:
PV =
g R T___
MM
Rearranging to solve for molar mass:
MM =
g R T___
PV
Finally, the experimental molar mass of butane will be compared to the theoretical molar
mass of butane.
PRELAB
1. What is butane’s chemical formula?
2. Calculate the theoretical molar mass of butane.
3. Write the balanced reaction that occurs when butane is burned in oxygen. Will
this reaction occur during this lab?
4. State Dalton’s law of partial pressures and discuss why it is necessary for this
lab.
MATERIALS
125 ml Erlenmeyer flask
glass plate
water trough
butane lighter
thermometer
ACADEMIC CHEMISTRY
100 ml graduated cylinder
wax pencil
PROCEDURE
NAME:
Collecting the butane:
1. Fill an Erlenmeyer flask completely full and cover the mouth of the flask with a
glass plate, making sure that no air gets trapped in the flask.
2. Fill a trough about ¾ full of water. Hold the plate in place and quickly invert the
flask into the water trough. If air bubble result, you must repeat this procedure.
3. Obtain a lighter and wipe it clean with a paper towel. Blow out any water the may
be trapped in the striker mechanism from a previous experiment. If the lighter is
adjustable, make sure the flame adjustment is set to give the largest flame.
4. Place the clean, dry lighter on a balance and determine its weight to the nearest
hundredth of a gram. Record this value in your data table.
5. Carefully hold the butane lighter underneath the Erlenmeyer flask. Make sure
that the gas opening of the lighter is beneath the mouth of the flask. Press the
release lever, being sure that the gas bubbles enter the flask.
6. Continue to hold the lever down until you have collected about 120 ml of gas.
7. Remove the butane lighter and let it dry completely on a paper towel.
8. Carefully raise or lower the Erlenmeyer flask until the water level inside the flask
is at the same point as the water level outside the flask. This it to make sure that
the total pressure inside the flask = atmospheric pressure (outside the flask).
Mark the gas/ water level using a wax pencil or a sharpie.
9. Remove the flask from the trough and fill it with water to the mark. Measure the
volume of the water using a graduated cylinder. This is really the volume of the
gas that was collected.
10. Thoroughly dry the lighter with a paper towel, weigh it, and record its mass in the
data table.
11. Measure the water temperature and record it.
12. Record the barometric pressure in the room in your table.
POST LAB QUESTIONS
1. Calculate the molar mass of an unknown gas if 975 ml of it at 19C and 0.962
atm weighs 7.29g.
2. A tank of compressed air has a total pressure of 8547 mmHg. If the partial
pressure of the nitrogen is 6,752 mmHg, and the partial pressure of the carbon
dioxide is 9 mmHg, what is the partial pressure of the oxygen?
ACADEMIC CHEMISTRY
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3. Discuss how the following errors would affect your overall molar mass
calculation:
a. The lighter still contained water when you weighed it the second time?
b. Some gas escaped into the water trough instead of entering the flask?
c. The vapor pressure of water was not accounted for?
DATA
Initial mass of lighter
Final mass of lighter
Mass of butane collected
Volume of the butane collected
(volume of water in the Erlenmeyer flask)
Liters of butane collected
Temperature of the water
Kelvin temperature of the water (and gas)
Pressure of water vapor at that temperature (see text
book appendix or Dalton’s law notes)
Barometric pressure
Pressure of the “dry” butane gas
Experimental molar mass of butane
Theoretical molar mass of butane
% error
CALCULATIONS
1. calculate the mass of butane collected
2. calculate the Kelvin temperature
3. calculate the pressure of the dry gas using Dalton’s law
ACADEMIC CHEMISTRY
4. molar mass calculation using ideal gas law
5. % error calculation
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