Ideal gas, collisions

Pressure Exerted by an Ideal Gas Collisions with a Wall... 

As a first approximation, it is reasonable to consider a cluster of i molecules as a sphere of radius r, and surface area A-jirf. The capture rate can be estimated simply as the frequency of collisions between the cluster and the monomer background gas times the probability a,- that a molecule that collides with the cluster will stick. The collision frequency is the surface area of the cluster, 4 7rrf, times the ideal gas collision rate per unit area,. ... 

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