1. 1/√(2) nπ d 2. 1/√(2) nπ d² 3. 1/√(2) n²π d² 4. 1/√(2) n²π² d²

Topic: Mean free path, Avogadro's number· Kinetic Theory of Gases

The mean free path for a gas, with molecular diameter d and number density n can be expressed as :

1. $\frac{1}{\sqrt{2}\,n\pi d}$

2. $\frac{1}{\sqrt{2}\,n\pi d^2}$

3. $\frac{1}{\sqrt{2}\,n^2\pi d^2}$

4. $\frac{1}{\sqrt{2}\,n^2\pi^2 d^2}$

Doubtly Asked 30 Oct 2026 NEET UG 2020 Practice test neet Save WhatsApp Report

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Answer: 2. $\frac{1}{\sqrt{2}\,n\pi d^2}$

The standard result for the mean free path of a gas is

$$\lambda=\frac{1}{\sqrt2\,n\pi d^2}$$

It falls with number density $n$ (more molecules means more collisions) and with the collision cross-section $\pi d^2$. The other options have the wrong powers of $n$ or $d$. Answer: option 2.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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The mean free path for a gas, with molecular diameter d and number density n can be expressed as :

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