A gas mixture consists of 2 moles of O2 and 4 moles of Ar at temperature T. Neglecting all vibrational modes, the total internal energy of the system is

The internal energy of 2 moles of O2 atom is UO2=n1f12RT=2×52×RT=5RTThe internal energy of  4 moles of Ar atom is UAr=n2f2RT2=4×32×RT=6RT∴ The total internal energy of the system is U=UO2+UAr=5RT+6RT=11RT