We know that van der Waals equation for real gases is (P+
n2a
V2
) (V - nb) = RT At low pressure, when the sample occupies a large volume, then the intermolecular forces do not play a significant role and the gas behaves as perfectly ideal. Constants a and b are called van der Waals constant and their value depends on the characteristic of a gas. Value of a is a measure of magnitude of intermolecular attractive forces within the gas and is independent of temperature and pressure. Pideal = Preal+
an2
V2
From the above equation, we can see that pressure exerted by real gas is lower than the pressure exerted by the ideal gas.