The total internal energy of one mole of HCl molecule is (assume...

HCl is a diatomic molecule.

For a non-rigid diatomic molecule, the degrees of freedom are:
Translational degrees of freedom = $3$
Rotational degrees of freedom = $2$
Vibrational degrees of freedom = $2$ (since each vibrational mode contributes two degrees of freedom, one for kinetic energy and one for potential energy).

Total degrees of freedom $f = 3 + 2 + 2 = 7$.

By the law of equipartition of energy, the internal energy of one mole of a gas is given by $U = \dfrac{f}{2}RT$.

Substituting $f = 7$, we get $U = \dfrac{7}{2}RT$.

Answer: $\dfrac{7}{2}RT$