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If \(q_{t}\) and \(Q_{t,m}\) are the molecular and molar translational partition functions of X2, respectively, then \(\ln(Q_{t,m}) =\)

(N is the Avogadro number)

  • A.\(N\ln q_{t} - N\ln N\)
  • B.\(N\ln q_{t} - \ln N\)
  • C.\(N\ln q_{t} + N\ln N + N\)
  • D.\(N\ln q_{t} - N\ln N + N\)

Answer: D

\(\ln Q_{t,m} = N\ln q_{t} - N\ln N + N\).

The molecules of a gas are indistinguishable, so the molar partition function is

\[ \begin{aligned} Q_{t,m} &= \frac{q_{t}^{N}}{N!} \\ \ln Q_{t,m} &= N\ln q_{t} - \ln N! \\ &\approx N\ln q_{t} - N\ln N + N \end{aligned} \]

using Stirling's approximation \(\ln N! \approx N\ln N - N\). Dropping the +N term gives the first option, writing \(\ln N\) for \(\ln N!\) gives the second, and the third has the wrong sign on \(N\ln N\).

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