The equation of state for an ideal gas can be applied to real gases as long as we limit the range of temperatures and pressures.

The "constant" in the equation obviously depends on the quantity of gas in the container. It also depends on the type of gas; oxygen, hydrogen etc., because, for a given mass of gas we have a different number of particles for different gases.

Avogadro suggested that at a given temperature and pressure, equal volumes of any gas (behaving as an ideal gas) contain equal numbers of particles. This is called Avogadro’s law and has been confirmed by experiment.

Therefore, if we consider a given number of particles of any gas in our cylinder we can find a really constant constant! This is called the universal gas constant, R.

The number of particles we chose to define this constant is (approximately) 6×10²³. This number is called Avogadro’s number, N_A. If we have this number of particles of a substance, we say we have 1mol of that substance.

The equation of state is therefore usually written as
 

where, n is the number of mols of gas.

The units of R are JK^-1mol^-1

It is often found useful to consider individual molecules of a gas. Fo this reason, we define the gas constant per molecule (or Boltzmann’s constant), k, as follows

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• The Equation of State of an Ideal Gas
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