From the measured molar heat capacities and the equipartition theorem,for a diatomic gas molecule the number of degrees of freedom from rotational motion are

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According to the equipartition theorem, the total energy of a molecule is equally distributed among all degrees of freedom. For a diatomic gas molecule, there are three translational and two rotational degrees of freedom, since the molecule can rotate about two perpendicular axes. However, for a diatomic gas at room temperature, only the vibrational and translational degrees of freedom are excited because the rotational energy levels are too far apart to be populated. Therefore, the molar heat capacity at constant volume is measured to be 5/2 R, which corresponds to the three translational and two vibrational degrees of freedom. Thus, the answer is 2 (D).

From the Measured Molar Heat Capacities and the Equipartition Theorem,for