Calculating the Molecular mass of a gas


The purpose of this experiment is to measure the volume and mass of a sample of butane, and to use these values to determine the molar mass.


Filling an inverted measuring cylinder with a known mass and volume of butane gas it is possible to calculate the molecular mass of butane; this can be done either by using the ideal gas equation or the assumption that a mole of gas will occupy 24dm3 at rtp. or 22.7dm3 at stp.


  • 500cm3 measuring cylinder
  • Tray or trough filled with water
  • Butane gas with delivery tube
  • Balance with an accuracy of 0.01g
  • Retort stand and clamp


  1. Fill a measuring cylinder with water and then invert into the bucket or trough of water. It is important not to allow any air into the measuring cylinder.
  2. Clamp the measuring cylinder in place to prevent it from falling over.
  3. Weigh a canister of butane camping gas and record in your results table.
  4. Use a delivery tube connected to the gas outlet by a small piece of silicone aquarium air hose add approximately 400cm3 of gas to the inverted measuring cylinder. Record the exact volume in your results table.
  5. Remove the deliver tube from the canister and re-weigh, record this value in your results table.
Mass of canister at start, m1 g
Mass of canister at the end, m2 g
Mass of butane gas, m1-m2 g
Exact volume of butane,  dm3                               m3
Atmospheric pressure, Pa  Pa


From the volume of gas added to the measuring cylinder calculate the number of moles of gas introduced. This can be done in the following two ways:

  1. Volume (dm3)/molar gas volume = n

At stp molar gas volume = 22.7 dm3

At rtp molar gas volume = 24.0 dm3


  1. Using the ideal gas equations:


Volume V is measured in meters cubed, m3

Gas constant R = 8.31 J K-1 mol-1

Temperature T measured in Kelvin, K

Pressure p is measured in pascals, Pa. A reasonably accurate atmospheric pressure value can be obtained using a google search of atmospheric pressures in your area or by using a phone app. Often these values are given as hectopascals. To convert hectopascals to pascals simply multiply by 100.

Once you have calculated the number of moles the molecular mass can be found by using:

Molecular mass=mass of gas/number of moles


  1. What value does the experiment give for the relative molecular mass of butane?
  2. Calculate the density of butane at r.t.p. from your results.
  3. What is the percentage difference between the molecular mass calculated using the ideal gas equation and the molar gas volume equation?