Van der Waals' equation for a real gas may be written as:

(Where, a, b, and R are the characteristic constants of particular gas, 'p' is the absolute pressure of the gas and 'v' represents the volume per unit mass).

Explanation:​

Real gases:

• The ideal gas law is only an approximation to the actual behaviour of the gases.
• At high densities that is at high pressure and low temperature the behaviour of the actual or real gases deviate from that predicted by the ideal gas law.
• In general at sufficiently low pressure or at low densities all gases behave like ideal gases.
• The ideal gases follow the characteristic gas equation and real gases follow the Van der Waals equation of state.

The ideal gas equation neglects the intermolecular forces and the volume occupied by the gas molecules. Vanderwall's gas equation takes into account these effects. Therefore real gasses follow Van der Waal's gas equation 

​\(\left( {P + \frac{a}{{{V^2}}}} \right)\left( {v - b} \right) = RT\)​

The constants a and b have positive values and are characteristic of the individual gas. The van der Waals equation of state approaches the ideal gas equation when values of these constants approach zero.

• The constant a provides a correction for the intermolecular forces (force of cohesion)
• Constant b is a correction for finite molecular size and its value is the volume of one mole of the atoms or molecules.​