IB Chemistry - Stoichiometry

IB Chemistry home > Syllabus 2025 > Stoichiometry > Mass and moles relationships in gases

If the number of moles present in a gas and its mass is known, then the relative molecular mass of a gas can be calculated.

Syllabus reference

Structure 1.5.3 - The molar volume of an ideal gas is a constant at a specific temperature and pressure.

  • Investigate the relationship between temperature, pressure and volume for a fixed mass of an ideal gas and analyse graphs relating these variables.

Guidance

  • The names of specific gas laws will not be assessed.
  • The value for the molar volume of an ideal gas under standard temperature and pressure (STP) is given in the data booklet.

Tools and links

  • Nature of science, Tool 2 and Tool 3, Reactivity 2.2 - Graphs can be presented as sketches or as accurately plotted data points.
  • What are the advantages and limitations of each representation?

Relative molecular mass calculation

This was one of the first means of establishing relative masses. The number of moles can be ascertained using the volume data at STP and the mass of the gas can be worked out from density or other, more direct, weighing methods.

Example: Calculate the relative molecular mass of a gas if 0.254g of it occupies 80cm3 at STP

1 mole of gas occupies 22.7 dm3 at STP

therefore 80 cm3 of gas = 3.52 x 10-3 moles

3.52 x 10-3 moles of gas has a mass = 0.254g

Therefore relative molecular mass = 0.254/(3.52 x 10-3) = 72.1
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