What percentage of nitrogen is present in dry air?

• A. 78%
• B. 21%
• C. 0.9%
• D. 0.04%

Answer: (A)

Think in terms of what dry air is made of by volume. The atmosphere is mostly nitrogen, so the largest portion by mole fraction is nitrogen, about 78% of dry air. The rest is mainly oxygen (about 21%), with small amounts of other gases—argon around 0.9%, carbon dioxide about 0.04%, and trace amounts of neon, helium, methane, krypton, hydrogen, and others. Water vapor is excluded when we say “dry air,” so humidity doesn’t change these percentages. Because nitrogen is by far the most abundant gas, 78% is the best answer.