[UPSC CSE 2015] Thermodynamics (Q80, 10M)

A Van der Waals gas undergoes Joule-Kelvin expansion with a pressure drop of 50\ \mathrm{atm}. If its initial temperature is 300^{\circ}\mathrm{K}, determine its final temperature. (Given Van der Waals constant a=0.136\ \mathrm{Pa\,m^6\,mol^{-1}}, b=36.5\times10^{-6}\ \mathrm{m^3\,mol^{-1}}, C_p=30\ \mathrm{J\,K^{-1}\,mol^{-1}}, R=8.3\ \mathrm{J\,K^{-1}\,mol^{-1}}.)