A cubic equation of state developed by van der Waals, which was an early attempt to describe the behaviour of real gases. The equation allows for the reality of the volume occupied by the gaseous molecules (V–b) where b is a constant appropriate for each gas, and attractive forces between the gaseous molecules, a. Allowing for these two corrections, van der Waals’ equation is given by:

where p is the pressure of the gas, V is the molar volume, R is the universal gas constant, and T is the absolute temperature.