The Molar Mass of Butane
Objective: To determine the molar mass of butane using the Ideal Gas Law
Procedure:
1. Fill a plastic pail about 2/3 full of water. Place a butane lighter under water for about 30 sec. Dry the lighter
with a paper towel and shake it several times to remove the water. Determine and record the mass of the
lighter in the data table.
2. Fill a 250-mL or 500-mL graduated cylinder completely with water, cover it with a glass plate and
carefully invert it into the pail of water. Do not allow any water to run out of the cylinder while you are
inverting it. If air bubbles appear in the cylinder, remove it from the water and repeat the procedure until
there are no air bubbles. Stand the cylinder in the pail.
3. Keeping the mouth of the cylinder under water at all times, raise the cylinder off the bottom of the pail, and
place the butane lighter under the mouth of the inverted cylinder. Allow gas bubbles to bubble up into the
cylinder until you have collected about 200 mL. Make sure that all the butane from the lighter is collected
in the cylinder. (you get better results if collecting about 200 ml in a 250 mL or 500 mL cylinder)
4. Remove the lighter from the pail, shake it to remove any excess water, dry it as you did before, and find the
mass of the butane lighter recording the result in the data table.
5. Adjust the level of the graduated cylinder so that the water level inside the cylinder matches the water level
in the pail. This is done to insure that the pressure inside the cylinder equals the atmospheric pressure. Now
measure the exact volume of gas collected, and record the result (in mL).
6. Measure the temperature of the water in the pail. We will assume that this is also the temperature of the
butane gas. Record the barometric pressure.
DATA
1
Mass of butane lighter, initial, g
g
2
Mass of butane lighter, final, g
g
m 3
Mass of butane gas collected, g
(#1 - #2)
g
V
4
Volume of butane gas collected, L
(___ mL/1000)
L
5
Barometric pressure,
(_____ mm Hg / 760)
6
Temperature of water and butane, K
( ___ C + 273)
7
Vapor pressure of water at temperature (#6)
atm
8
Partial pressure of butane (#5 - #7)
atm
T
P
atm
K
Calculation of the Molar Mass of Butane:
1. Use the Ideal Gas Equation: PV = nRT; PV = gRT/mw; Therefore, mw = gRT/PV where R =.0821 L
atm/mol K) Show your work below. Circle your answer.
2. The chemical formula for butane is C4H10. What is the theoretical molar mass for butane?
3. Comparing the molar mass of butane that you determined to the theoretical molar mass (question 1
above). Calculate your error and percent error for this lab exercise.
a) error = observed value - theoretical value =
b) % error = (100 x error) / theoretical value =
4. Do an error analysis of the results.