Explanation

(a)
The exact value of internal energy is not known as it includes all type of energies of molecules constituting the given mass of matter such as translational, vibrational and rotational. The kinetic and potential energy of the nuclei and electron with in the individual molecules and the manner in which the molecule are linked together, are
$E = E_{translational} + E_{rotational} + E_{vibrational}$
Thus, we can say that internal energy is partly potential ad partly kinetic