Question

ideal gas .450 mole initial pressure 16 atm and 290 K expands isothermally to a final pressure 1 atm. find work , if expansion is against a vacuum, a constant external pressure of 1 atm and reviersibly

GK

W(isothermal) = nRT*ln(V2/V1)
or
W(isothermal) = nRT*ln(P1/P2)

See also:
http://www.diracdelta.co.uk/science/source/i/s/isothermal%20expansion/source.html