Two moles of an ideal gas expand isothermally at 400 K from 5.0 L to 15.0 L. (a) Calculate the work done by the gas. (b) If the same expansion were isobaric at the initial pressure, would the work be larger or smaller? Solution: (a) W = n R T ln(V2/V1) = (2)(8.31)(400) ln(15/5) = 6648 x ln 3 = 6648 x 1.0986 = 7304 J, about 7.3 kJ. (b) Initial pressure P1 = nRT/V1 = (2)(8.31)(400)/0.005 = 1.33 x 10^6 Pa. Isobaric work = P1(V2 - V1) = (1.33 x 10^6)(0.010) = 13,300 J - larger, because the isobar stays at the high initial pressure instead of falling along the hyperbola.