Van der Waals Equation
A modified equation of state that corrects the ideal gas law for molecular volume and intermolecular attraction.
Formula / equation
(p + an²/V²)(V − nb) = nRT
Unit
Various
In depth
The constant a measures the strength of intermolecular attraction and raises the effective pressure, while b represents the excluded volume of the molecules themselves. Large values of a indicate a gas that liquefies easily, and the equation reproduces the S-shaped isotherms near the critical point.
Examples in the real world
Water vapour has a large a (5.54 L²·atm·mol⁻²) reflecting hydrogen bonding; helium's is only 0.034.