Using the van der Waals equation of state, derive an expression that gives the isothermal change of internal energy with volume. Use this expression to calculate the change in internal energy with volume. Then use this expression to calculate the change in internal energy when 1 g-mole of nitrogen is expanded from a density of 0.23 g/cm^3 to 0.02 g/cm^3 at 220 K. Express your answer in calories.