Measure the volume and the mass of a gas from a pressurised cylinder by collecting the gas over water and calculating its relative molecular mass.
In this experiment, students observe as a sample of butane from a small pressurised cylinder is collected over water in a measuring cylinder. By measuring the volume of the gas, and determining its mass by weighing the cylinder before and after collection, students can calculate the relative molecular mass of the butane.
This is most likely to be done as a teacher demonstration. Teachers of advanced students may wish to consider the possibility of a student practical, but would need to carry out very careful risk assessments in the context of the capabilities of their students.
Although the most convenient gas for this is butane, other gases may be available in similar small cylinders.
The collection of a gas sample, and the weighing of the gas cylinder before and after this, should take about 5–10 minutes as a demonstration.
Source: Royal Society of Chemistry
How to set up the equipment required for collecting a sample of butane to measure the volume of the gas
There are two possible routes for using the results of the experiment to calculate the RMM:
Investigating the nature of the propellants in aerosol cans is essentially an extension of this experiment. The composition of most present-day aerosol propellants is a mixture of butane isomers and propane. The varying proportions of propane and butane will lead to a measured RMM between those of propane (46) and butane (58). By measuring the apparent RMM, the proportions of propane and butane can be calculated. However, the other ingredients of the aerosol may affect the measurements.
Oxygen (oxidising) is also available in small pressurised canisters – see CLEAPSS Hazcard HC069. Teachers adapting this experiment for measuring the RMM of oxygen will need to consult their employer’s risk assessments, but this may be a preferred alternative to butane for use as a class experiment. However, the purity of the oxygen in these cylinders may be significantly less than 100%, which will affect the RMM value obtained.
This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.
© Nuffield Foundation and the Royal Society of Chemistry