For each molecule, the kinetic energy associated with each degree of freedom is [itex]\frac{1}{2}kT[/itex], so the kinetic energy of each molecule in the O2, N2 mixture is [itex]\frac{5}{2}kT[/itex]. Summing over the entire gas gives the internal energy [itex]U = N\frac{5}{2}kT = nN{_\mathrm{A}} \frac{5}{2}kT = \frac{5}{2}nRT[/itex].