A gaseous mixture consists of $2$ moles of oxygen and $4$ moles of argon at temperature $T$. Neglecting all vibrational modes, the total internal energy of the mixture is
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Use $U = nC_vT$, and remember diatomic = $\dfrac{5}{2}R$, monatomic = $\dfrac{3}{2}R$ when vibrational modes are ignored.