Solution

ID: past-exam-of-the-chemistry-a-course-of-the-university-of-cambridge/2025/paper-2/a7/b/ii/solution

Use the same standard pressure and temperature, the same bond length, and the same electronic and vibrational-ground-state approximations. The translational partition function scales as mass to the power , giving the factor . For the original isotopologue, and the rotational symmetry number is two. For the isotope-substituted molecule, the two oxygen ends are distinguishable, so the symmetry number is one. Hence
The change in standard molar entropy is
so
This uses a convention omitting a separate nuclear-spin entropy. If nuclear-spin degeneracies are instead included explicitly, the spin- carbon-13 nucleus adds a further relative to the spin-zero original isotopologue; that convention must be stated rather than silently mixed with the quoted partition functions.

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