IIT JEE Advanced 2008 Paper 1

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.