\begin{aligned} & \mathrm{C}_{\mathrm{p}}-\mathrm{C}_{\mathrm{v}}=\mathrm{R} \Rightarrow \mathrm{C}_{\mathrm{v}}=\left(\mathrm{C}_{\mathrm{p}}-\mathrm{R}\right) \\ & \mathrm{C}_{\mathrm{v}}=6 \mathrm{cal} / \mathrm{mol}^{\circ} \mathrm{C} \\ & \Delta \mathrm{U}=n \mathrm{C}_{\mathrm{v}} \Delta \mathrm{~T} \\ & =5 \times 6 \times 10=300 \mathrm{cal} \end{aligned}