Compressibility factor

z is the compressibility factor. For ideal gases z = 1. But the fact that this value is not a constant for real gases is revealed by experimental data.

The graph of the product PV against P for different gases at the temperature 273 K

• According to this graph it is clear that real gases approach ideal behaviour under conditions of low pressure.

The graph of PV/RT against P for a mole of nitrogen at various temperatures.

It is clear from the above graph that at high temperatures the real gases approach ideal behaviour.

Van der Waals equation

P = Pressure

V = Volume

n = Amount (moles) of substance

R = Universal gas constant

T = Absolute temperature a and b are constants (Van der Waals constants) for real gases

(n²a)/V²  is the correction for pressure drop due to Van der Waals interactions. nb is the correction for the volume of gas molecules.