Question:

Initially, the root mean square (rms) velocity of \(\mathrm{N_2}\) molecules at certain temperature is \(u\). If this temperature is doubled and all the nitrogen molecules dissociate into nitrogen atoms, then the new rms velocity will be :

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Use \(u_{rms} = \sqrt{3RT/M}\): doubling T and halving M changes the speed by \(\sqrt{2\times2}\).
Updated On: Nov 11, 2026
  • \(u/2\)
  • \(2u\)
  • \(4u\)
  • \(14u\)
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The Correct Option is B

Solution and Explanation

\(u = \sqrt{\frac{3RT}{M}}\) for \(\mathrm{N_2}\) with \(M = 28\).
After dissociation \(\mathrm{N_2} \rightarrow 2\mathrm{N}\) the molar mass becomes 14 and \(T' = 2T\).
\(u' = \sqrt{\frac{3R(2T)}{14}} = \sqrt{\frac{3RT}{28}\times4} = 2u\).
Answer: (2) \(2u\)
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