The ideal gas equation describes the behaviour of an ideal gas. An ideal gas is one where its behaviour will follow the kinetic theory or model. This is a model developed by scientists to help explain the properties and behaviour of gases. It should be noted that the kinetic theory is a model of how an ideal gas will behave. It has a number of assumptions, these include:

- Gases consist of particles that move in a totally random way.
- The volume of the particles in a gas are negligible when compared the total volume of the gas. That is gases are mostly empty space.
- There are no intermolecular or attractive forces between the particles in a gas.
- The gas particles collide with each other and the walls of any container in totally elastic collision (that is one in which no kinetic energy is lost).
- The average kinetic energy of the gas particles is proportional to the temperature of the gas.

You should complete a few questions to ensure that you are confident and will be able to answer
any questions on the ideal
gas equation which appear in your exam. Check your understanding by doing clicking the link below to complete
the practice questions. The main problems students have with the
ideal gas equation is the units. The
exam board are likely to use the value of 8.31 JK^{-1}mol^{-1} for R, the gas constant.
This means that you must use the
following units for the other variable:

- The volume(V) must be in cubic metres (m
^{3}). It is highly likely that the volume in the exam is likely to be in ml/cm^{3}or dm^{3}. In this case you will need to convert these units into cubic metres. Remember there is 1000 litres or 1000dm^{3}in a cubic metre. There are 1 million cubic centimetres (cm^{3}) in a cubic metre. - The temperature(T) must be in degrees Kelvin.
- The pressure(p must be in pascals. If you are given a pressure in kPa (kilopascals) then you must convert this into pascals (Pa).

You maybe asked in the exam to calculate the M_{r}
of an unknown gas or volatile liquid using the ideal gas equation.
This simply
requires a bit of simple arithmetic to rearrange the ideal gas equation.

We already know that:

One method which could be used is described below in the diagram.

So using a simple gas syringe and an accurate sensitive balance we can find m (the mass of the gas), temperature
will simply be room temperature, pressure will be atmospheric, R is 8.31, the gas constant. So all we have to do is
rearrange the ideal gas equation from above:

Rearranging to make M