The van der Waals equation [local] is simply a modification of the ideal gas equation that accounts for deviations in ideal gas behavior at extreme conditions of temperature and pressure.

Bridgeman van der Waals site.

Ideal Gas Equation

van der Waals Equation

The coefficients a and b added to the equation account for:

• a: Corrects for intermolecular attractive forces
• b: Corrects for volume of individual gas molecules

The constant a (a measure of attractive forces) is directly proportional to the boiling temperature.