Cv of air is 20.785 J/mol.K. Please explane clearly

5.36 A reversible heat pump transfers heat from the air contained in a rigid tank A and rejects heat to the air contained in a rigid tank B as shown in the figure below. Initially, both tanks contain 0.05kmol of air at 200kPa and 800K. The heat pump operates until the air temperature in tank A drops to 650K. a) Calculate the final temperatures and pressures of tanks A and B. b) Calculate the work input to the heat pump. (Answer: a) PA=162.5kPa,TB=984.6K,PB=246.2kPa, b) 35.9kJ )