A furnace that provides heat by burning methane gas (CH4) must have the correct mixture of air and fuel to operate efficiently. What is the mole ratio of air to methane gas in the combustion of methane? Air is 20% oxygen.

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CH4 + 2O2 - - > CO2 + 2H2O

Then the ratio is 1mol CH4 / 2 mol pure O2

But, air is 0.2 mol of pure O2 / mol of air.

Multiply the last ratio by 10/10 = > 2mol of pure O2 / 10 mol of air.

=> 10 mol air / mol CH4 ... this is the answer