The van der Waals equation is and a derivation is given in the following Justification. The equation is often written in
terms of the molar volume Vm = V/n as. The constants a and b are called the van der Waals coefficients. As can be understood
from the following Justification, a represents the strength of attractive interactions and
b that of the repulsive interactions between the molecules. They are characteristic of
each gas but independent of the temperature (Table 1.6). Although a and b are not
precisely defined molecular properties, they correlate with physical properties such as
critical temperature, vapor pressure, and enthalpy of vaporization that reflect the
strength of intermolecular interactions. Correlations have also been sought where
intermolecular forces might play a role. For example, the potencies of certain general
anaesthetics show a correlation in the sense that a higher activity is observed with
lower values of a (Fig. 1.17).