Calculate the change in the entropies of the system and the surroundings, and the total change in entropy, when the volume of a sample of argon gas of mass 21 gat 298 K and 1.50 bar increases from 1.20 dm3 to 4.60 dm3 in
(a) An isothermal reversible expansion,
(b) An isothermal irreversible expansion against Pex = 0, and
(c) An adiabatic reversible expansion.