Molar heat capacity of (deuterated form of formaldehyde) vapour at constant pressure is vapour 14 ca — Thermodynamics and Thermochemistry Chemistry Question
Question
Molar heat capacity of $\text{CD}_2\text{O}$ (deuterated form of formaldehyde) vapour at constant pressure is vapour 14 cal/K-mol. The entropy change associated with the cooling of 3.2 g of $\text{CD}_2\text{O}$ vapour from 1000 K to 900 K at constant pressure is (assume ideal gas behaviour for $\text{CD}_2\text{O}$) [$\ln 0.9 = -0.1$]
Answer: B
💡 Solution & Explanation
Molar mass of $\text{CD}_2\text{O} = 12 + 2(2) + 16 = 32 \text{ g/mol}$. Moles $n = \frac{3.2}{32} = 0.1 \text{ mol}$. Entropy change $\Delta S = n C_p \ln\left(\frac{T_2}{T_1}\right) = 0.1 \times 14 \times \ln\left(\frac{900}{1000}\right) = 1.4 \times (-0.1) = -0.14 \text{ cal/K}$.
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