question_answer 24)

                  Diatomic molecules like hydrogen have energies due to both translational as well as rotational motion. From the equation in kinetic theory \[pV=\,\frac{2}{3}\,E,\,E\] is                 (a) the total energy per unit volume.                 (b) only the translational part of energy because rotational energy is very small compared to the translational energy.                 (c) only the translational part of the energy because during collisions with the wall pressure relates to change in linear momentum.                 (d) the translational part of the energy because rotational energies of molecules can be of either sign and its average over all the molecules is zero.

Answer:

                  (c) The answer (c) is self-explanatory. Note that E is the internal energy of translation, rotation and vibration of the molecules. As the molecules of an ideal gas can be treated as point masses, kinetic energies due to rotational and vibration are zero. Thus, E is exclusively translational in character.