According to the law of equipartition of energy, every degree of freedom has average enerrgy of
per molecule.
There are 3 degrees of freedom corresponding to translational motion, 3 corresponding to vibrational motion.
So the average energy corresponding to transational motion only is
When we also consider the vibrational motion of the molecule, the body has both transalational as well as vibrational motion, and each type of motion contributes 3 degrees of freedom.
Hence average energy per molecule when vibration is considered(in addiition to translation) is
