Lab 8: Determination of the Molar Mass of Butane by Vapor Density
Introduction:
Butane lighters are one of the most common forms of lighters that are currently on the market. The lighter is
filled with butane and from this a flame can be induced with a spark. To determine the molar mass of butane
using the vapor density method, one must conduct an experiment where the total mass and number of moles of
butane can be found. By measuring and recording he mass of pocket lighter before and after releasing butane
gas, the mass can be calculated. The ideal gas law sates that:
PV = nRT
We can use this to determine the moles of butane gas present if its volume, partial pressure, and temperature are
known.
To find the volume, butane gas will be collected over water in an inverted graduated cylinder. When the water
levels are the same inside and outside the cylinder, the pressure inside the cylinder will be equal to the pressure
outside the cylinder. Thus, measuring atmospheric pressure in the room will provide you with the total pressure
above the water inside the cylinder. The volume can only be measured when the two water levels are in line
with one another so that the volume can be an accurate representation of butanes volume.
There are two gases that account for the total pressure inside the cylinder: butane and water vapor. Dalton’s law
of partial pressures states that the total pressure inside the system is simply the sum of pressures of each of the
gases, so for this lab:
Ptotal = Pwater + Pbutane
You will know the total pressure (atmospheric pressure) and the partial pressure of water based on the
temperature of the water. From this you will be able to calculate the pressure of butane.
Once all these values are known (temperature, pressure, and volume), then the moles of gas released can be
obtained. The molar mass of butane is then determined by dividing the mass of butane released by the number
of moles of butane released:
Mass of Butane
Molar Mass =
Moles of Butane
Materials:
 Butane pocket lighter

 100 mL graduated cylinder
 Thermometer
 Water bucket
 Balance
 Paper towel
 Hair dryer (if available)
Procedures:
1. Clean and dry the butane lighter. Use a hair dryer to help you dry the lighter if one is available. Measure and
record the mass of the lighter. Do not release any gas from the lighter after you have measured this. If you do
release gas, then you will need to re-mass the lighter.
2. Obtain an orange bucket that contains water.
3. Immerse the 100 mL graduated cylinder in the water. Once you have removed all the air bubbles inside the
graduated cylinder, invert the cylinder so that its mouth remains under the surface of the water. Examine the
bottom of the graduated cylinder, which is now pointed upwards, to see if any air bubbles are present. Repeat
the step until no air bubbles are present.
4. Being careful to keep the mouth of the cylinder under water, immerse the lighter in the water trough, and
position it so that the released gas bubbles will be trapped in by the cylinder.
5. Once the lighter is completely under the graduated cylinder, depress the lever that releases the butane gas.
You will see gas bubbles ascending up the graduated cylinder and collecting in the cylinder. Continue to hold
down the lever until you have collected at least 90 mL of gas. Don’t go over 100 mL!
6. Making sure that the mouth of the cylinder remains under the surface of the water, raise and lower the
cylinder until the water level on the inside matches the water level outside. Record the volume on the cylinder
when you reach this level.
7. Dry the lighter with paper towels or a hair dryer.
8. Measure and record the final mass of the lighter.
9. Measure the temperature of the water in the orange bucket of water
Data:
Mass of Lighter Before Experiment
Mass of Lighter After Experiment
Mass of Butane Released
Volume of Gas Collected (mL)
Volume of Gas Collected (L)
Atmospheric Pressure (on board)
Water Temperature
Calculated Vapor Pressure (torr)
Calculations: Due 1 week from today!
1. Determine the partial pressure of butane gas using the formula:
Ptotal = Pwater + Pbutane
2. Determine the moles of butane gas collected
3. Calculate the experimental molar mass using the equation:
Mass of Butane
Molar Mass =
Moles of Butane
4. The molecular formula for butane is C4H10. Calculate the actual molar mass of butane.
5. Determine the percentage error in your experimentally determined value for the molar mass of butane:

%Error 
Actual Value - Experimental Value
Actual Value
Analysis: Due 1 week from today!
1. Does your percent error indicate that the results you obtained were accurate? Justify your reasoning!
 with the way that the experiment designed and suggest how these problems could be
2. Suggest three problems
corrected so that better results could be obtained. NOTE – Human error is not an acceptable response.
3. Assume you did not take into account the vapor pressure of the water in the graduated cylinder. How would
this impact your final molar mass? Be sure to explain thoroughly.