2) (a) Natural gas is burned in a burner. The dry exhaust products from combustion with air contain 5% oxygen, 9% carbon dioxide and 86% nitrogen by mass. Note that the "dry exhaust products" means that the water vapour in the products is condensed before this measurement. Assuming complete combustion, find the excess air used for this burner. (b) If the enthalpy of vaporization of butane is 22.4 kJ/kg fuel at 298K, what are the upper and lower heating values of liquid butane? Data: Enthalpy of formation of water at 298K = -241, 847 kJ/kmol Enthalpy of vaporization of water at 298K = 44,010 kJ/kmol Formation enthalpy of O₂ and N₂ are zero at 298 K Formation enthalpy of CO2 at 298K = -393,546 kJ/kmol Formation enthalpy of butane at 298K = -124,700 kJ/kmol