The pressure depends on both the frequency of collisions with the walls and the
force of each collision. Both the frequency of the collisions and their force are
reduced by the attractive interactions, which act with a strength proportional to
the molar concentration, n/V, of molecules in the sample. Therefore, because both
the frequency and the force of the collisions are reduced by the attractive interactions,
the pressure is reduced in proportion to the square of this concentration.
If the reduction of pressure is written as −a(n/V)2, where a is a positive constant
characteristic of each gas, the combined effect of the repulsive and attractive forces
is the van der Waals equation of state as expressed in eqn 1.21.