IB Chemistry - Stoichiometry

IB Chemistry home > Syllabus 2025 > Stoichiometry > The equation of state

As has been shown by Boyle's law and Charles' law, the behaviour of gases conforms (to a close approximation) to some simple relationships between the gas volume, pressure and temperature for a given amount of gas. Combining Boyle's law and Charles' law gives the equation of state.

Syllabus reference

Structure 1.5.4 - The relationship between the pressure, volume, temperature and amount of an ideal gas is shown in the ideal gas equation PV = nRT and the combined gas law P1V1/T1= P2V2/T2.

  • Solve problems relating to the ideal gas equation.


  • Units of volume and pressure should be SI only. The value of the gas constant R, the ideal gas equation, and the combined gas law, are given in the data booklet.

Tools and links

  • Tool 1, Inquiry 2 - How can the ideal gas law be used to calculate the molar mass of a gas from experimental data?

The equation of state

The equations of Boyle's law (PV = constant) and Charles' law (V/T = constant) may be combined to give the expression: PV/T = constant. This is called the equation of state. This new combined gas equation can be used to find the new volume, pressure and temperature of a gas if the conditions are changed.

The equation of state:

ColSol Testing