GATE 2017 ME (ME1) – Question 46
Air contains 79% nitrogen and 21% oxygen on a molar basis. Methane burns completely with 50% excess air. Find the molar percentage of nitrogen in the products.
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Correct answer: 72 to 75
Explanation
For one mole of methane, stoichiometric oxygen is 2 moles. The supplied 50% excess air contains 3 moles of oxygen and $3(79/21)=11.2857$ moles of nitrogen. Wet products are $1CO_2+2H_2O+1O_2+11.2857N_2$, totaling 15.2857 moles. Nitrogen percentage is $100(11.2857/15.2857)=73.83\%$. The key uses wet products, including water vapour.