Air expands in a piston-cylinder device with a polytropic exponent of 1.3. The initial pressure and temperature were 630 KPa and 550oC respectively. The final pressure is 100 kPa. Find the specific entropy change of the gas for the process. Air can be assumed to be an ideal gas( with average values of "Cp" and "R" as 1.005 kJ/kg K and 0.287 kJ/kg K respectively).