The internal energy per molecule for a diatomic (non-rigid) gas is (where k_B is Boltzmann's constant and T is absolute temperature).

Step-by-step solution

For a non-rigid diatomic gas, the molecules possess translational, rotational, and vibrational motion. The degrees of freedom are distributed as follows: Translational degrees of freedom = 3 Rotational degrees of freedom = 2 Vibrational degrees of freedom = 2 (one for kinetic energy and one for potential energy) Total degrees of freedom f = 3 + 2 + 2 = 7 According to the law of equipartition of energy, the average energy associated with each degree of freedom per molecule is 1 2 k_B T . Internal energy per molecule