 Solution

The integer molecular mass of the stated isotopologue isTake $p^{\circ }=1\mathrm{bar}=10^5\mathrm{Pa}$, the recommended standard pressure.^[ref]

At $298\mathrm{K}$, the thermal de Broglie wavelength is $\Lambda \simeq 9.3100\times 10^{-12}\mathrm{m}$. The result of part (a) givesIf $1\mathrm{atm}$ is used instead, the result decreases by $R\ln (1.01325)\simeq 0.1094\mathrm{J}{\mathrm{K}}^{-1}\mathrm{mo}{\mathrm{l}}^{-1}$, giving $168.23$ in the same units. Specifying the pressure removes that small convention dependence.