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The chemical potential \(\mu\) for an indistinguishable ideal monoatomic gas molecule is \(\mu\) = ..... (where, \(\beta = 1/(k_{B}T)\)) :

  • A.\(-k_{B}T\Big[\ln\frac{V}{N} + \frac{3}{2}\ln\frac{2\pi m}{\beta h^{2}} + 1\Big]\)
  • B.\(-k_{B}T\Big[\ln\frac{V}{N} + \frac{3}{2}\ln\frac{2\pi m}{\beta h^{2}}\Big]\)
  • C.\(-k_{B}T\Big[\ln\frac{V}{N} + \frac{3}{2}\ln\frac{2\pi m}{\beta h^{2}} + 5/2\Big]\)
  • D.\(-k_{B}T\Big[\ln V + \frac{3}{2}\ln\frac{2\pi m}{\beta h^{2}} + 1\Big]\)

Answer: B

The chemical potential is \(-k_{B}T\left[\ln\frac{V}{N} + \frac{3}{2}\ln\frac{2\pi m}{\beta h^{2}}\right]\).

For an ideal gas of indistinguishable particles, \(Z = z^{N}/N!\) with \(z = V(2\pi m/\beta h^{2})^{3/2}\). Using Stirling's approximation, \(F = -Nk_{B}T[\ln(z/N) + 1]\), and

\[ \begin{aligned} \mu &= \left(\frac{\partial F}{\partial N}\right)_{T,V} \\ &= -k_{B}T\ln\frac{z}{N} \end{aligned} \]

which expands to the keyed expression. The \(+1\) in \(F\) disappears on differentiating with respect to \(N\).

The option with \(+1\) is \(F/N\), the free energy per particle, not \(\mu\). The option with \(+5/2\) is the Sackur-Tetrode entropy per particle divided by \(k_{B}\). The option with \(\ln V\) alone has lost the \(1/N\) that comes from the \(N!\) of indistinguishable particles, so it is not even intensive.

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