NCERT Solutions
Class 11 Physics
Kinetic Theory

Q. 12.10
Estimate the mean free path and collision frequency of a nitrogen molecule in a cylinder containing nitrogen at 2.0 atm and temperature 17 0C. Take the radius of a nitrogen molecule to be roughly 1.0 Å. Compare the collision time with the time the molecule moves freely between two successive collisions (Molecular mass of N2 = 28.0 u).
Given:
Mean free path = 1.11 × 10–7 m
Collision frequency = 4.58 × 109 s–1
Successive collision time ≈ 500 × (Collision time)
Pressure inside the cylinder containing nitrogen, P = 2.0 atm
= (2.026 × 105) Pa
Temperature inside the cylinder, T = 17°C =290 K
Radius of a nitrogen molecule, r = 1.0 Å = (1 × 1010) m
Diameter, d = (2 × 1 × 1010) = (2 × 1010) m
Molecular mass of nitrogen, M = 28.0 g = (28 × 10–3) kg
The root mean square speed of nitrogen is given by the relation:
vrms=
Where,R is the universal gas constant = 8.314 J mole–1 K–1
Therefore vrms=
=508.26m/s
The mean free path (l) is given by the relation:
l=
Where, k is the Boltzmann constant = (1.38 × 10–23) kgm2 s–2K–1
Therefore l= = 1.11 × 10–7 m
Collision frequency=
=
= (4.58 × 109) s–1
Collision time is given as:
T=
= = 3.93 × 10–13 s
Time taken between successive collisions:
T’= =
m/s = 2.18 × 10–10 s
=
= 500
Hence, the time taken between successive collisions is 500 times the time taken for a collision.