In this Justification we have built the van der Waals equation using vague arguments
about the volumes of molecules and the effects of forces. The equation can be
derived in other ways, but the present method has the advantage that it shows how
to derive the form of an equation from general ideas. The derivation also has the
advantage of keeping imprecise the significance of the coefficients a and b: they
are much better regarded as empirical parameters that represent attractions and
repulsions, respectively, rather than as precisely defined molecular properties.