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Chemistry of the Environment: Question 6

Syllabus 10.3

Multiple choice Core 1 mark

Clean, dry air is a mixture of gases, but two elements make up the vast majority of it by volume.

Which option correctly gives the approximate percentages by volume of these two gases, with the remainder made up of a mixture of noble gases and carbon dioxide?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the composition of clean, dry air

Clean, dry air is approximately 78%78\% nitrogen (N2\text{N}_2) and 21%21\% oxygen (O2\text{O}_2) by volume. The remainder, only about 1%1\%, is a mixture of noble gases (mostly argon) and carbon dioxide (CO2\text{CO}_2).

Step 2: Why the other options are wrong

  • B: swaps nitrogen and oxygen. Oxygen is the smaller of the two components, not the larger.
  • C: nitrogen and oxygen are not present in equal amounts; nitrogen makes up far more of the air than oxygen.
  • D: neither oxygen nor carbon dioxide is the largest component. Carbon dioxide is only part of the tiny remainder, and nitrogen (not mentioned in this option) is actually the biggest single component of air.

Final answers

  • Clean, dry air is 78%\boxed{78\%} nitrogen and 21%\boxed{21\%} oxygen by volume, option A.