Rankers Physics
Topic: Thermal Physics
Subtopic: Kinetic Theory of Gases

A gas mixture consists of 2 moles of $O_2$ and 4 moles of $Ar$ at temperature T. Neglecting all vibrational modes, the total internal energy of the system is: (2017-Delhi)
$15\text{ RT}$
$9\text{ RT}$
$11\text{ RT}$
$4\text{ RT}$

Solution:

$O_2$ is diatomic ($f=5$), $U_1 = n_1\frac{f_1}{2}RT = 2 \times \frac{5}{2}RT = 5RT$. Ar is monoatomic ($f=3$), $U_2 = n_2\frac{f_2}{2}RT = 4 \times \frac{3}{2}RT = 6RT$. Total energy $U = U_1 + U_2 = 5RT + 6RT = 11RT$.

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