For a diatomic gas with all vibrational modes excited, the molar specific heat at constant volume is:
A$(3/2)R$
B$(5/2)R$
C$(7/2)R$
D$R$
Answer & Solution
Correct answer: C. $(7/2)R$
DOF: 3 trans + 2 rot + 2 vib (KE+PE) = 7. $C_V = (7/2)R$. $C_P = (9/2)R$. $\gamma = 9/7 \approx 1.29$. (At room temperature, vibrations are typically frozen out.)
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