Energy of all molecules of a monoatomic gas having a volume V and pressure P is 3/2 PV. The total translational kinetic energy of all molecules of a diatomic gas at the same volume and pressure is : 

3/2 PV

energy of 1 mol of gas : E = \(\frac{f}{2}RT = \frac{f}{2}PV\)

... where f : degree of freedom

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

\(\Rightarrow E = \frac{3}{2}PV\)