An ideal gas contained in a cylinder with a volume of 500m3 is at a pressure of 400Kpa and a temperature of 70 ?C . Pressure in the cylinder is dropped to 200KPa by releasing some gas. If the expansion process of the cylinder is isentropic, find the mass of the gas remaining in the cylinder. Note that the specific heat ratio of the gas is 1.4 and the gas constant is 0.287KJ/KgK.