Van der Walls noted that the total force of attraction on any molecule about to hit a wall is proportional to the concentration of neighbouring molecules, n/V. However, the number of molecules about to hit the wall per unit wall area is also proportional to the concentration n/V. Therefore, the per unit wall area, or pressure,is reduced from that assumed in the ideal gas wall by a factor proportional to n2/V2. Letting a be the proportionality constant, we can write
P(actual)=P(ideal)−n2a/V2
P(ideal)=P(actual)−n2a/V2