Natural gas is burned to produce hot water to heat a clothing store. Assuming that the natural gas can be approximated as methane (CH₄) and that the air is a simple mixture of O₂ and N₂ in molar proportions of 1:3.76, determine the following:

A. The molar air–fuel ratio for stoichiometric combustion.

B. The mass air–fuel ratio for stoichiometric conditions.

C. The operating air–fuel ratio (mass) of the heater when 4% (molar or volume) oxygen, O₂, is present in the flue gases.

D. The percent excess air, the percent theoretical air, and the equivalence ratio associated with the conditions given in part C.