Which gas is the third most abundant by volume in dry air?

Which gas is the third most abundant by volume in dry air?

• A. Neon
• B. Krypton
• C. Carbon Dioxide
• D. Argon

Answer: (D)

The key idea is that the volume percentage of gases in dry air matches their mole fractions in the mixture. In dry air, the major components are nitrogen and oxygen, with argon as the next most abundant gas. Roughly, nitrogen is about 78%, oxygen about 21%, and argon around 0.93%. Carbon dioxide is only about 0.04%, while neon and krypton exist only in trace amounts (much less than 0.01%). Since argon’s fraction (~0.93%) is far larger than the others listed besides nitrogen and oxygen, it ranks as the third most abundant by volume in dry air. Water vapor would alter these numbers in humid air, but not the ranking among these gases. Argon’s persistence is helped by its inert nature, so it remains present in that ~1% range.

The key idea is that the volume percentage of gases in dry air matches their mole fractions in the mixture. In dry air, the major components are nitrogen and oxygen, with argon as the next most abundant gas. Roughly, nitrogen is about 78%, oxygen about 21%, and argon around 0.93%. Carbon dioxide is only about 0.04%, while neon and krypton exist only in trace amounts (much less than 0.01%). Since argon’s fraction (~0.93%) is far larger than the others listed besides nitrogen and oxygen, it ranks as the third most abundant by volume in dry air. Water vapor would alter these numbers in humid air, but not the ranking among these gases. Argon’s persistence is helped by its inert nature, so it remains present in that ~1% range.