Molar Mass of Butane Lab
Purpose: To determine the molar mass of a gas.
Pre-lab discussion: In this experiment you will collect a volume of butane gas and use
the ideal gas equation to determine the molar mass of the gas. The pressure of the gas
collected will be equalized with the surrounding ambient air pressure. Water
temperature will be measured to determine the vapor pressure of water present in the
gas mixture collected. This can be subtracted out to determine the pressure of the dry
butane alone. Room temperature, atmospheric pressure and gas volume will be
measured and substituted into the ideal gas equation to determine the number of
moles in the sample collected. By comparing the initial mass of the lighter assembly
to the final mass, the mass of the gas collected can be determined. Knowing both the
number of moles and grams collected allows for calculation of molar mass.
Equipment and Materials:
 barometer
 butane lighter
 electronic balance (milligram),  0.001 g
 graduated cylinder, 100. mL
 thermometer
 water bin
Procedure:
1. Find the mass of a lighter on a milligram electronic balance.
2. Submerge a 100. mL graduated cylinder in water so that the cylinder fills
completely with water. Invert the cylinder, ensuring that there are no air
bubbles remaining inside.
3. Hold the lighter in the water under the cylinder and depress the button to
dispense butane gas into the lighter. Butane is very insoluble in water, so it
will bubble up into the cylinder and displace the water.
4. Release enough butane to fill the tube to within 5 mL of its graduated capacity.
5. Adjust the level of the water inside and outside the tube to be equal. This
makes the pressure of the combined water vapor and butane gas equal to the
pressure of the atmosphere. While the pressure is the same inside the tube as it
is outside, record the volume to the nearest mL.
6. Measure and record the barometric pressure in the room from the PASCO GLX
(as determined by the weather sensor).
7. Measure and record the temperature of the water bath using a digital
thermometer.
8. Thoroughly dry the lighter and measure the mass again.
9. Repeat steps 1-8 for at least two additional trials.
10. Use the data collected and the ideal gas equation to determine the molar mass
of butane.
11. Determine your percentage error, based on the accepted molar mass of butane,
or C4H10.
Observations and Data:
A
Measurement
Mass of lighter assembly and contents
before collection (± _______ g)
B
Mass of lighter assembly and contents
after collection (± _______ g)
C
Mass of butane gas evolved
(A - B) (± _______ g)
D
Volume of gas produced at room
temperature and atmospheric pressure
(± _______ cm3)
Barometric pressure (inches Hg)
(read by Pasco weather sensor attached
to GLX, (± _______ inches Hg)
Barometric pressure (mm Hg)
(E x 25.4 mm/in) (± _______ mm Hg)
E
F
G
H
I
J
K
Trial 1
Trial 2
Trial 3
Water bath temperature
(read by Pasco weather sensor attached
to GLX, (± _______ C)
Vapor pressure of water at this
temperature (mm Hg) (from online
calculator)
Web URL:__________________________
_________________________________
Partial pressure of dry butane
Ptot = Pbutane + Pwater vapor
(F – H) (± _______ inches Hg)
Calculated molar mass of butane with
uncertainty
Percentage error
Calculations & Propagation of Uncertainty:
For one trial, label each step in words and show propagation of uncertainty throughout.
Calculate separate, but Mrs. Wright doesn’t need to see the work for the other two trials.
Conclusions and Evaluation: (must be typed up)
Follow lab write-up handout and IB rubric for writing a conclusion.