Molecular Mass of Butane
The Avogadro hypothesis states that at standard temperature and pressure (STP), the volume of 1.00 moles
of any gas is 22.4 L. In this experiment you will use this definition to calculate the molecular mass of butane, but this
method can be used to measure the molecular mass of any gas.
Materials:
250 mL Erlenmeyer flask
butane lighter
analytical balance
glass plate
thermometer
barometer
trough or bucket
graduated cylinder
watch glass
Safety:
Butane is flammable. Use care with the lighters and keep experimental apparatus away from flames.
Procedure
1.
Choose a lighter. Wipe it dry with a paper towel.
2.
Accurately weigh the lighter using the balance. Record this mass in your table.
3.
Fill a 250 mL Erlenmeyer flask completely with water. No air bubbles are allowed—they will affect the results.
Cover the top with the watch glass.
4.
Fill a pneumatic trough or other suitable container with water. Holding the watch glass with your finger, carefully
turn the flask over and place the neck under the surface of the water in the trough. Carefully remove the watch
glass, making sure that no air bubbles enter the flask at any time. If they do, you must repeat this step.
5.
Carefully hold the butane lighter under the water. Make sure that the gas opening is beneath the mouth of the
inverted flask. Press the release lever, being sure that the gas bubbles enter the flask (see fig. 1).
6.
Continue to hold the lever down until you have collected approximately 150-200 mL of the gas.
7.
Remove the butane lighter, dry it with a paper towel, and set it aside.
8.
DO NOT remove the flask from the water. Carefully raise or lower the flask until the water level inside the flask is
equal to the water level outside the flask. This is done to equalize the pressure inside and outside the flask. At
this point, slide the watch glass over the opening of the flask to trap the water in the flask. You may remove the
flask once the levels are equal.
9.
Using a graduated cylinder, measure the volume of the trapped water. Record this number in your table. Fill the
flask completely with water and measure this volume using your graduated cylinder. Record this value also.
10. Accurately weigh the dry lighter again. Be sure to remove ALL water from the lighter. Record this mass.
11. Measure and record the temperature of the water in the trough and the barometric pressure in the room in atm
and torr.
Questions
1.
2.
3.
What is the purpose of equalizing the water levels inside and outside the flask?
If you did not use the vapor pressure of water, what effect would this have on your final molecular weight?
Some gases cannot be measured by this method. Describe some characteristics of these gases.
Data
Mass of lighter before experiment:
_________________________
Mass of lighter after experiment:
_________________________
Mass of butane:
_________________________
Volume of the flask:
_________________________
Volume of trapped water:
_________________________
Volume of butane collected:
_________________________
Temperature of water bath:
_________________________
Pressure (atmospheric):
_________________________
Vapor pressure of water:
_________________________
Pressure of dry butane:
_________________________
The molecular mass of dry butane can be calculated by knowing the mass of the collected gas and the ideal gas law.
Since molecular weight equals grams per mole (gmoles), we can solve the ideal gas law for n and divide the mass
of the collected dry butane (m) by this number:
Molecular Weight = (mRT)(PV)
Calculate the molecular weight of butane from your data and find the theoretical weight (its formula is C 4H10). Show
all work. Circle your final answer. Determine your percent error.
Fig. 1