Mean Free Path, Diffusion, and Effusion of Gases

9 Mean Free Path, Diffusion, and Effusion of Gases 

The average distance that a molecule travels between collisions is its mean free path. At room temperature and atmospheric pressure, the mean free path of a nitrogen molecule with a molecular diameter of 300 pm (four times the covalent radius) is 93 mn, or about 310 molecular diameters. If the nitrogen molecule were the size of a golf baU, it would travel about 40 ft between collisions. Mean free path increases with decreasing pressure. Under conditions of ultrahigh vacuum (10 ° torr), the mean free path of a nitrogen molecule is hundreds of kilometers. 

The process by which gas molecules spread out in response to a concentration gradient is diffusion, and even though the particles undergo many collisions, the root mean square velocity still influences the rate of diffusion. Heavier molecules diffuse more slowly than lighter ones, so the first molecules you smell from a perfume mixture (in a room with no air currents) are the lighter ones. 

The average distance between collisions is the mean free path. 

The ratio of effusion rates of two different gases is given by Graham s law of effusion, named after Thomas Graham (1805-1869)  

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