A sample of dry air has a temperature of 273K, a volume of 4 m^3 and a pressure of 2 atmospheres. The air parcel then undergoes the following changes: a) warmed under constant pressure to 400K, b) cooled under constant volume to 200K, c) cooled under constant pressure to 100K, and d) warmed under constant volume to 300K. Sketch the process on a P-V diagram and mark the final values for pressure and volume after each change. Calculate the total work done.