What is the approximate molar ratio of nitrogen to oxygen in air used in combustion calculations?

What is the approximate molar ratio of nitrogen to oxygen in air used in combustion calculations?

• A. 3.76
• B. 1.0
• C. 2.0
• D. 4.5

Answer: (A)

Air is composed of about 79 mole percent nitrogen and 21 mole percent oxygen when we treat it as dry. That means for every mole of O2, there are roughly 79/21 ≈ 3.76 moles of N2 accompanying it. In combustion calculations we use this ratio to account for the nitrogen that comes with the oxidizer, since N2 is mostly inert in the flame and simply adds to the total gas volume and affects exhaust composition. So the approximate molar ratio of nitrogen to oxygen in air used in these calculations is about 3.76 to 1.

Air is composed of about 79 mole percent nitrogen and 21 mole percent oxygen when we treat it as dry. That means for every mole of O2, there are roughly 79/21 ≈ 3.76 moles of N2 accompanying it. In combustion calculations we use this ratio to account for the nitrogen that comes with the oxidizer, since N2 is mostly inert in the flame and simply adds to the total gas volume and affects exhaust composition. So the approximate molar ratio of nitrogen to oxygen in air used in these calculations is about 3.76 to 1.