An ideal gas, ( C _ v = (5)/(2) R ), is expanded adiabatically...

An ideal gas, (\overline{\mathrm{C}}_{\mathrm{v}}=$\frac{5}{2} \mathrm{R}$), is expanded adiabatically against a constant pressure of 1 atm untill it doubles in volume. If the initial temperature and pressure is (298 $\mathrm{~K}$) and (5 $\mathrm{~atm}$), respectively then the final temperature is _______ ($\mathrm{K}$) (nearest integer). [(\overline{\mathrm{C}}_{\mathrm{v}}) is the molar heat capacity at constant volume]