Question:

The molar heat capacity (\(\mathrm{C_p}\)) of \(\mathrm{CD_2O}\) is 10 cals at 1000 K. The change in entropy associated with cooling 32 g of \(\mathrm{CD_2O}\) vapour from 1000 K to 100 K at constant pressure will be: (D = deuterium, atomic mass = 2u)

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Find moles from the molar mass of CD2O (12 + 2x2 + 16 = 32 g/mol), then use \(\Delta S = nC_p\ln(T_2/T_1)\).
Updated On: Nov 11, 2026
  • \(23.03\ \mathrm{cal\,deg^{-1}}\)
  • \(-23.03\ \mathrm{cal\,deg^{-1}}\)
  • \(2.303\ \mathrm{cal\,deg^{-1}}\)
  • \(-2.303\ \mathrm{cal\,deg^{-1}}\)
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The Correct Option is B

Solution and Explanation

Molar mass of \(\mathrm{CD_2O}\) = 12 + 2(2) + 16 = 32 g/mol, so 32 g = 1 mol.
At constant pressure \(\Delta S = nC_p \ln\frac{T_2}{T_1}\).
\(\Delta S = 1 \times 10 \times \ln\frac{100}{1000} = 10 \times (-2.303) = -23.03\ \mathrm{cal\,deg^{-1}}\).
Answer: (2) \(-23.03\ \mathrm{cal\,deg^{-1}}\)
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