Physics · Kinetic theory of gases · chapter 12 of 14

Class 11 Physics Chapter 12: Kinetic Theory Assertion-reason questions with answers

2 assertion-reason questions on Kinetic Theory, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): At the same temperature, hydrogen molecules have a greater rms speed than oxygen molecules. Reason (R): At a given temperature the average kinetic energy per molecule is the same for all ideal gases, so lighter molecules move faster.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both are true. ½m v̄² = (3/2)kBT is the same for H₂ and O₂ at the same T; with smaller m, H₂ must have the larger vrms. R is exactly why A holds.

Question 2
Assertion (A): Cp − Cv has the same value for helium and for nitrogen. Reason (R): A rigid diatomic molecule has five degrees of freedom.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both are true. Cp − Cv = R for any ideal gas, mono, di or polyatomic, which is why A holds. R correctly counts the degrees of freedom of N₂, which fixes Cv = (5/2)R, but it does not explain why the difference Cp − Cv is common to both gases.

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