A sample of methane gas of mass 4.5 g has a volume of 12.7 dm^3 at 310K. It expands isothermally against a constant external pressure of 200 Torr until its volume has increased by 3.3 dm^3. Assuming that methane is a perfect. Part A. Calculate work, heat and U. Part B. Calculate work, heat and U assuming expansion carried out reversibly.