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Official Soldier Technical Paper: [Lansdowne] - 28 Feb 2021

Option 1 : H_{2}

The correct answer is **H**__ 2__.

__Key Points__

**Mean free path:**- According to Classics, every gas comprises an infinite number of molecules that are perfectly elastic spheres.
**The size of the gas molecules is tiny as compared to the distance between them**.- Further, the molecules of a gas in the state of never-ending, speedy, and random motion; undergo perfectly elastic collisions with one another.
- As the molecules exert no force on one another except during collision, the free path traveled between two successive collisions will be a linear path with invariant velocity.
**Therefore, the path of a single molecule consists of a series of short zig-zag paths of different lengths.**- These paths of varying lengths are called free paths of the molecules, and their mean is called the mean free path.
**The mean free path is inversely proportional to molar mass. So, the smaller the molar mass, the higher the mean free path.****So as hydrogen has the smallest molar mass, so it has the largest mean free path. Hence, Option 1 is correct.**