NEET 2026 (Phase 1) · PhysicsRMS speedPrevious Year Question
A flask contains argon and chlorine in the ratio of 2 : 1 by mass. The temperature of the mixture is 27 °C. The ratio of root mean square speeds of the molecules of the two gases (v_rms^Ar / v_rms^Cl) is: (Atomic mass of argon = 40.0 u and molecular mass of chlorine = 70.0 u)
Answer: (A) √7 / 2. Correct Answer (A): √7 / 2 (= √(7/4)) The rms speed is v_rms = √(3RT/M).
- A.√7 / 2✓
- B.√7 / 4
- C.7/2
- D.2/7
Correct Answer
(A) √7 / 2
Solution & Explanation
Correct Answer (A): √7 / 2 (= √(7/4)) The rms speed is v_rms = √(3RT/M). At the same temperature it depends only on molar mass: v_rms ∝ 1/√M. The 2 : 1 mass ratio of argon to chlorine is a distractor — it does not enter the rms-speed ratio. Hence v_rms^Ar / v_rms^Cl = √(M_Cl/M_Ar) = √(70/40) = √(7/4) = √7/2. The official key marks option (1), whose intended value is √7/2 (equivalently √(7/4)); option B is written as √7/4 which is a different (incorrect) quantity.
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