In air used for combustion calculations, the mole ratio of nitrogen to oxygen is commonly expressed as what value?

In air used for combustion calculations, the mole ratio of nitrogen to oxygen is commonly expressed as what value?

• A. 3.76
• B. 1.00
• C. 0.78
• D. 14.7

Answer: (A)

Air used in combustion problems is treated as having about 21% oxygen and 79% nitrogen by moles. That makes the nitrogen-to-oxygen mole ratio 0.79/0.21 ≈ 3.76. So for every mole of O2 involved, about 3.76 moles of N2 come along with it. This ratio is standard because it accounts for the inert nitrogen that dilutes the reacting gases and affects flue-gas composition and heat balances. The other numbers don’t fit the actual proportions of air: for example, 1.00 would imply equal moles of N2 and O2, 0.78 isn’t the correct N2/O2 ratio in air, and 14.7 is atmospheric pressure, not a mole ratio.

Air used in combustion problems is treated as having about 21% oxygen and 79% nitrogen by moles. That makes the nitrogen-to-oxygen mole ratio 0.79/0.21 ≈ 3.76. So for every mole of O2 involved, about 3.76 moles of N2 come along with it. This ratio is standard because it accounts for the inert nitrogen that dilutes the reacting gases and affects flue-gas composition and heat balances. The other numbers don’t fit the actual proportions of air: for example, 1.00 would imply equal moles of N2 and O2, 0.78 isn’t the correct N2/O2 ratio in air, and 14.7 is atmospheric pressure, not a mole ratio.