Estimate the mean free path and collision frequency of a nitrogen molecule in a cylinder containing nitrogen at 2.0 atm and temperature 17 °C. 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).

First we have to find the rms speed of the nitrogen molecule, then by finding the collision time and the time the between two successive collisions, and comparing the two by dividing the latter by former we get the required result.


Given:


Mean free path = 1.11 × 10–7 m


Collision frequency = 4.58 × 109 s–1


Pressure inside the cylinder containing nitrogen (P)


P = 2 atm = 2.026 × 105 Pa


Temperature inside the cylinder (T) = 17°C = 290 K


Radius of a nitrogen molecule (r) = 1 Å = 1 × 1010 m


Diameter (d) = 2r


d = 2 × 1010 m


Molecular mass of nitrogen (M) = 28 g


M = 28 × 10–3 kg


The root mean square speed of nitrogen is given by


Vrms =


Vrms =


Vrms = 508.26 m/s


Let the mean free path be L, given as


L =


L =


L = 1.11 × 10-7 m


Collision frequency (f) = Vrms /L


f = 508.26 / (1.11 × 10-7 )


f = 4.58 × 109 s-1


The Collision time is given as (T)


T = d / Vrms


T = (2 × 10 -10 ) / (508.26)


T = 3.93 × 10-13 s


Time taken between successive collisions be Tc


Tc = l / Vrms


Tc = (1.11 × 10-7 ) / (508.26)


Tc = 2.18 × 10-10 s


Tc / T = (2.18 × 10-10) / (3.93 × 10-13)


Tc / T = 500


Hence, the time taken between successive collisions is 500 times the time taken for a collision.


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