Physics Heat & Thermodynamics Level: Misc Level

Whenever a gas expands -when we exhale, when a flask is opened, and so on -the gas undergoes an increase in entropy. A sample of methane gas of mass 25 g at 250 K and 185 kPa expands isothermally and (a)reversble, (b) ireversible until its pressure is 2.5 kPa. Calculate the change in entropy of the gas.