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Question:

The gas mixture consists of 3 moles of oxygen and 5 moles of argon at temperature T. Considering only translational and rotational modes, the total internal energy of the system is:

12 RT

20 RT

15 RT

4 RT

Solution:

U=f1/2n1RT+f2/2n2RT=5/2(3RT)+3/2×5RT
U=15RT