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At room temperature, the free path of molecules present in air is approximately-
1. 10-7 /m
2. 10-4 /m
3. 10-10 /m
4. 10-13 /m

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Correct Answer - Option 2 : 10-4 /m


Mean Free Path (λ): 

  • The distance travelled by a gas molecule between two successive collisions is known as a free path.
  • During two successive collisions, a molecule of gas moves in a straight line with constant velocity.
  • The average distance that a particle can travel between two successive collisions with other particles.

\(\lambda = \frac{{Total\;distance\;travelled\;by\;a\;gas\;molecule\;between\;successive\;collisions\;\;}}{{Total\;number\;of\;collisions}}\)

\(\Rightarrow \lambda = \frac{1}{{\sqrt 2 \pi n{d^2}}}\)


n = number of molecules per unit volume and

d = diameter of a molecule

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