\( 10 \) mole of oxygen is heated at constant volume from \( 30^\circ\text{C} \) to \( 40^\circ\text{C} \). The change in the internal energy of the gas is __________ \( \text{cal} \). (The molecular specific heat of oxygen at constant pressure, \( C_p = 7 \, \text{cal} / \text{mol}^\circ\text{C} \) and \( R = 2 \, \text{cal}./\text{mol}.^\circ\text{C} \))
