Calculate work done when pressure of gas is decreased

Question

Response to the following problem:

(a) Show that the work done by an ideal gas during the quasi-static, isothermal expansion from an initial pressure P_i to a final pressure P_f is given by

W = nRT In P_f/P_i

(b) Calculate the work done when the pressure of 1mol of an ideal gas is decreased quasi-statically from 20 to 1 atm, the temperature remaining constant at 20°C (R = 8.31 llmol· deg).