Q.

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    [2017]

1 15 RT  
2 9 RT  
3 11 RT  
4 4 RT  

Ans.

(3)

The internal energy of 2 moles of O2 atom is

      UO2=n1f12RT=2×52RT=5RT

The internal energy of 4 moles of Ar atom is

       UAr=n2f2RT2=4×32RT=6RT

   The total internal energy of the system is

        U=UO2+UAr=5RT+6RT=11RT