NCERT Class 11 Physics Solutions: Chapter 2-Unit and Measurement Part 10

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Question: 2.17

One mole of an ideal gas at standard temperature and pressure occupies 22.4 L (molar volume). What is the ratio of molar volume to the atomic volume of a mole of hydrogen?

(Take the size of hydrogen molecule to be about). Why is this ratio so large?

• Volume of hydrogen atom

• Now, 1 mole of hydrogen contains hydrogen atoms.

Volume of 1 mole of hydrogen atoms,

• Molar volume of 1 mole of hydrogen atoms at STP,

• Hence, the molar volume is times higher than the atomic volume. For this reason, the inter-atomic separation in hydrogen gas is much larger than the size of a hydrogen atom.

Question: 2.18

Explain this common observation clearly: If you look out of the window of a fast moving train, the nearby trees, house etc. seem to move rapidly in a direction opposite to the train’s motion, but the distant objects (hill tops, the moon, the stars etc.) seem to be stationary. (In fact, since you are aware that you are moving, these distant objects seem to move with you).