Van der Waal’s equation of state Van der Waal modified the equation of state for real gases, pV=RT. He incorporated in the equation corrections due to the non zero attractive force between the molecules and the finite volume of the molecules.

The intermolecular attraction tends to reduce the actual pressure imparted by the gas molecules on the walls of the container. The pressure correction is therefore proportional to the number of molecules hitting the container wall per second, that is the number of molecule per unit volume. Again each molecule hitting the wall of the container is attracted by other molecules in its vicinity. This is also proportional to the number density. Therefore p is replace by

p ® p+constt.r ²

or, p ® p+ a/V²

Due to the finite volume of the molecules the actual volume available to the gas molecules is V-b, where b is volume of the gas molecules per mole. Therefore,

V® V-b

The equation of state is therefore,

(p +a/V²)(V-b) = RT (v1)

Where a and b are constants.The values of the constants are found experimentally.