MOLAR MASS OF A GAS DETERMINED EXPERIMENTALLY
PRE-LAB DISCUSSION:
In this lab, we are going to determine the molar mass of butane, C4H10, experimentally. A simple
calculation using a periodic table would give us the correct answer for the molar mass of butane,
however, you are going to conduct an experiment in the lab to see how close you can come to
the accepted value. We know the idea gas law is PV = nRT. The ideal gas law equation can
be rearranged to solve for n, the number of moles:
n = PV
RT
We are trying to find the molar mass, which is expressed in units of grams per mole (g/mol). We
will measure the mass of the butane released from the pressurized container, and we will
measure the volume of the gas that we collect. You can use the method of water displacement at
room conditions and then substitute the measurements of volume, temperature, and pressure into
the ideal gas law equation in order to find n, the number of moles of butane. Once we know the
mass, and the number of moles, we will divide the mass by the number of moles to calculate the
molar mass (g/mol).
WARNING: Butane is a flammable gas, and at NO TIME during this lab should there be
any use of an open flame or other heat source!
QUESTION: What is the molar mass of butane? Butane is the gas used as fuel for a cigarette
lighter.
HYPOTHESIS: If we can capture a sample of gas and determine the mass of the sample and the
number of moles, then we can calculate the molar mass using the following equation:
Molar Mass = mass of sample in grams
number of moles
PROCEDURE:
1. Determine the initial mass for the butane lighter provided by your instructor ( +/- .01 gram ).
2. Set up the water basin for collecting a gas in an inverted graduated cylinder by water
displacement. Use a 100 mL graduated cylinder.
3. Release butane from the pressurized container and collect a sample of gas with a volume of
approximately 50.0 mL. Collect every bubble that leaves the lighter. If you miss a bubble,
you will have to repeat the entire procedure beginning with re-weighing the clean and dry
lighter.
4. Adjust the cylinder up or down so that the water level inside the graduated cylinder is the same
as the level outside the cylinder. THIS IS VERY IMPORTANT! If you miss this simple step,
the pressures inside and outside the cylinder will not be equal, and you will have an
inaccurate estimation of the pressure inside the cylinder where the gas is collected.
5. Record the volume of butane that was collected in the Observations and Data section.
6. Measure and record the temperature of the water in the basin ( +/- 0.1 °C ). This will be used
to determine the vapor pressure of water H2O (g) in the graduated cylinder.
(a Table of water vapor pressure at a given temperature will be provided)
7. Obtain the barometric pressure in the lab from weather.com and then covert to atm.
8. Make sure the lighter is dry; then measure and record the mass of the lighter ( +/- .01 gram).
9. Clean and return all materials their proper place.
10. Report your measurements and calculations on the class data sheet.
OBSERVATIONS AND DATA:
Name
Group
This sheet will be turned in by each person at the end of the period for a grade.
Mass
1. Beginning mass of butane container (minitial )
grams
2. Final mass of butane container (mfinal )
grams
3. Mbutane = Mass of butane collected (line 1 – line 2)
grams
Volume
4. Volume of the gas collected
mL
5. Vbutane = Volume of butane, converted to liters (line 4 ÷ 1000)
L
Temperature
6. Temperature of water basin in °C
°C
7. Tbutane = Temperature of water in K
K
(we can assume the water temp = butane temp)
Pressure
(READ the instructions below)
8. Barometric pressure, according to weather.com
Ptotal
atm
*Note: weather.com reports pressure as inHg. Convert to mmHg, then to atm.
9. Vapor pressure of water at the temperature in line 6
(see Table 1)
Pwater
atm
Note: The gas collected in the cylinder is a mixture of C4H10 (g) and H2O (g).
Use Dalton’s Law of Partial Pressure to calculate the pressure of butane in the mixture.
Ptotal = P butane + P water vapor
10. Pressure of dry butane
Pbutane
atm
Moles
1. Calculate the number of moles of butane collected using R = 0.0821 atm · L / mol · K
PV = nRT
P=
V=
n = __________
R=
Τ=
n=
2. Calculate the experimental molar mass of butane by dividing the mass, M (line 3),
by n, the number of moles.
MM experimental =
3. Calculate the accepted value for the molar mass of butane using a periodic table.
Butane C4H10
MM accepted =
4. Calculate the ERROR (the difference between the accepted value and your
experimental value).
5. Calculate the percent error in your result. This is:
|𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 – 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙
x 100
% error
Analysis:
1. How many grams of butane did you collect in this experiment?
2. What volume would the same number of grams of ethane (C2H6) have occupied at the
same conditions of temperature and pressure?