Name:
Date:
AP Chemistry
Overview
The ideal gas law provides us with a tool for the determination of molar masses from physical measurements (mass, pressure, volume and temperature). In this lab, you will determine the molar mass of butane, C
4
H
10
, from a disposable lighter by recording the four physical measurements described above and using your knowledge of the ideal gas law. Since the gas will be collected over water, a correction will be made for the pressure caused by the water vapor in the sample of collected gas.
Materials
-butane lighter -graduated cylinders
-250 mL Erlenmeyer flask -glass/plastic plate
-plastic trough
-electronic balance
-barometer
-hair dryer
Safety
Butane gas is highly flammable and needs to be released in a fume hood or in a well ventilated space
Procedure
1.
Fill the 250 mL flask completely full with tap water. Cover the top with the glass plate without trapping any air in the top of the flask
2.
Fill the plastic trough ~ 2/3 full with water. Holding the glass plate in place with your finger, carefully turn the flask upside down and place it in the trough. BE SURE THAT NO
AIR BUBBLES ENTER THE FLASK. Remove the glass plate
3.
Immerse the butane lighter in the water in the trough and then remove it. Dry the lighter thoroughly with a paper towel. A hair dryer may also be used to dry the lighter.
Weigh the thoroughly dried lighter.
4.
Carefully hold the lighter under the water, making sure that the gas opening is beneath the mouth of the inverted flask. Lift the mouth of the flask slightly while keeping it under the water at all times. Press the release level of the light ensuring that all of the gas bubbles enter the flask. Continue to hold the level until you’ve collected ~200 mL of gas.
5.
Remove the light, shake off excess water and dry it in the exact same way you did earlier. Re-weigh the lighter to determine the mass of butane released.
6.
DO NOT REMOVE THE GAS-FILLED FLASK FROM THE WATER. Instead, carefully raise or lower the flask until the level of water inside the flask is the same as the level of water outside of the flask. This ensures that the gas inside the flask is at atmospheric pressure.
Hold the flask at this level and slip the glass plate under the flask. Holding the plate tightly in place with your finger, lift the flask out of the water and place it on your lab bench without spilling any water.
1

Name:
Date:
AP Chemistry
7.
Remove the glass plate in the fume hood or near the door to let the butane gas to escape. Then, using your graduated cylinders, fill the flask with water KEEPING COUNT
OF THE EXACT VOLUME OF WATER REQUIRED TO FILL THE FLASK EXACTLY TO THE TOP.
8.
Measure the temperature of the water in the plastic trough and record this as the temperature of the butane gas.
9.
Record the barometric pressure by looking up today’s air pressure online. Record this as the atmospheric pressure.
Data:
Mass of lighter before experiment
Mass of lighter after experiment
Mass of butane gas used
Volume of water required to fill flask
Volume of butane gas collected
Temperature of water
Temperature of butane gas
Atmospheric Pressure
Vapor Pressure of water (use table)
Corrected Pressure of Butane gas
Analysis/Calculations:
1.
Why does raising or lowering the flask until the level of water inside the flask is the same as the level of water outside of the flask ensure that the butane gas is at atmospheric pressure?
2

Name:
Date:
2.
Calculate the molar mass of butane from your experimental data
AP Chemistry
3.
Using the actual molar mass of butane (C
4
H
10
), determine the percent error for your experiment.
Error
4.
After removing the flask from the trough and filling it up with water, you don’t fill it all the way to the top. Would this make your final calculation for the molar mass higher or lower than expected? Explain.
5.
At the beginning of the experiment, you noticed that there were some air bubbles in your flask before you put it underwater in the trough. Would this make your final calculation for the molar mass higher or lower than expected? Explain.
6.
Generate your own source of error that is different form the two described above and explain if this would make your molar mass higher or lower than expected.
3