Energy of all molecules of a monoatomic gas having a volume V and pressure P is $\frac{3}{2}PV$ . The total translational kinetic energy of all molecules of a diatomic gas as the same volume and pressure is:

Energy of 1 mole of gas $=\frac{f}{2}RT=\frac{f}{2}PV$

where $f$ = Degree of freedom

Monoatomic or diatomic both gases possess equal degree of freedom for translational motion and that is equal to 3 $i.e. f=3$

$\therefore E=\frac{3}{2}PV$

Although total energy will be different,

For monoatomic gas $E_{total}=\frac{3}{2}PV [As f=3]$

For diatomic gas $E_{total}=\frac{5}{2}PV [As f=5]$