LAB:
Finding the Molar Mass of Butane
Materials:
• Volumetric Flask 500.0 mL
• Plastic bucket
• Butane lighter
• Electronic balance
• (Optional – Compressed air)
Procedure:
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Read this procedure and construct a data table that will accommodate the data you will
collect. Use a spread sheet program to produce the data table.
Find the mass of the butane lighter
Fill the bucket 2/3 full of warm tap water
Drop lighter into water and wait 5 min for it to equalize in temperature
During this 5 minute wait, obtain the 500.0 mL volumetric flask, fill it with water at the same
temperature as that in the bucket, measure and record the temperature of the water.
Stopper the flask and invert into bucket.
Remove the stopper and keep the opening of the flask under water.
Depress the lever of the lighter and allow the butane gas to bubble up into the flask.
BE PATIENT! THIS WILL TAKE AT LEAST SEVERAL MINUTES. You may need to take
turns with your partner holding the lighter and flask.
Fill the flask to the 500.0 mL mark.
Stopper the flask and remove it from the bucket.
Place the flask into the fume hood and remove the stopper to allow it to vent the butane
(butane is flammable of course).
Carefully and thoroughly dry the lighter and find it’s mass again. If compressed air is
available, a stream of high velocity air will aid in removing the water from the small crevices
of the lighter.
Determine the following values:
• Barometric pressure of the air in the room (internet weather info or weather channel)
• Vapor pressure of water at the temperature you measured (table provided).
Calculations:
(on your own paper)
1. Determine the mass of the butane collected
2. Determine the pressure of the butane ALONE in the flask (minus the water vapor).
3. Use the following formula to calculate the molecular weight of your butane:
Molecular weight (g/mol) =
mRT
PV
Where:
m = mass of gas
R = universal gas constant (62.4 mmHg x Liters x Kelvin-1 x mol-1 )
T = temperature of gas in Kelvins
P = pressure of gas in mmHg
V = volume of gas in liters
4. Write the chemical formula for butane
5. Using the periodic table, determine the theoretical (true) molecular weight for butane.
6. Calculate the % error in your lab results.
% error = (laboratory – theoretical)/theoretical x 100
Questions:
(on your own paper)
1. If the lighter was not completely dry before it was weighed the second time, in what way
would the calculated molecular weight of butane be affected?
2. If some of the bubbles of butane did not enter the flask and escaped into the air instead, in
what way would the calculated molecular weight of butane be affected?
3. If the temperature measurement of the water was too high, in what way would the calculated
molecular weight of butane be affected?
4. If the barometric pressure of the room was actually lower than the pressure used in your
calculation, in what way would the calculated molecular weight of butane be affected?
5. If you didn’t subtract the pressure of the water vapor from that of the wet butane gas, in what
way would the calculated molecular weight of butane be affected?
6. One variable in this experiment that was not controlled, discussed, or noticed is the solubility
of butane gas in water. If some of the butane gas dissolved into the water, in what way
would the calculated molecular weight of butane be affected?