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The volume of a perfect gas is doubled, the number of atoms N and the energy E being held constant, the change in entropy will be :

  • A.Nk ln V
  • B.2Nk ln V
  • C.Nk ln 2
  • D.½ Nk ln (2V)

Answer: C

The entropy change is Nk ln 2.

At constant N and energy E (free expansion of an ideal gas), only the volume term of the Sackur-Tetrode entropy changes: \(S = Nk\ln V + \) terms independent of V. So:

\[ \Delta S = Nk\ln\frac{2V}{V} = Nk\ln 2 \]

The answer depends only on the ratio of volumes, so options containing ln V or ln 2V alone cannot be a change in entropy.

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