Help with complete combustion ratios (not that hard of a question)?

Question

For a complete combustion, it takes 5 moles of pure O2 oxygen gas to react with every 2 moles of ethyne (C2H2). However, if air contains only 21% oxygen by volume, then what is the ratio of air to ethyne needed for a complete combustion? Would it be 2 moles of ethyne for 25 moles of air (since around 1/5 of air is oxygen, and 5 moles of oxygen is needed)?

Answer 1

You're on the right track. To refine your answer, you have to use the principle that equal volumes of gases contain equal numbers of molecules. So instead of moles of air, liters of air per 2 liters of acetylene.