4. (25 points) In a combustion chamber methane and ethane are burned with air. The stack gas (or flue) leaves at 300C and 105kPa. There is no CO, and no residual O2, methane or ethane in the stack gas (conversion is 100% ). The partial pressure of CO2 in the stack gas is 80mmHg. Find the mole fraction of methane in the fuel and the dew point temperature of the flue