Volume of 4 gases in dry air

Dry air is the mixture of gases in the atmosphere once the water vapour has been removed. We say dry air because the amount of water vapour changes from place to place and day to day (it depends on temperature and humidity), so it is not given a fixed value.

The four most abundant gases, and their approximate proportions by volume, are:

Gas	Formula	Approx. % by volume	Rough fraction
Nitrogen	N₂	≈ 78%	about four-fifths
Oxygen	O₂	≈ 21%	about one-fifth
Argon	Ar	≈ 1% (0.9%)	a small slice
Carbon dioxide	CO₂	≈ 0.04%	a tiny sliver

The big picture in plain numbers. If you imagine 100 identical boxes of air:

• about 78 boxes would be nitrogen,
• about 21 boxes would be oxygen,
• about 1 box would be argon,
• and just a small corner of one box (about 0.04) would be carbon dioxide.

Nitrogen is the most abundant gas — not oxygen. This is the single most common mistake. We breathe in oxygen to stay alive, so students assume air is "mostly oxygen". It is not: air is mostly nitrogen (≈ 78%), and oxygen is only about 21%.

The numbers are approximate. The examiner accepts close values: nitrogen 78% (or ~80%), oxygen 21% (or ~20%), argon ~1%, carbon dioxide ~0.04% (sometimes written ~0.03–0.04%).

Exam questions often show the composition of air as a pie chart or bar chart and ask you to identify a gas, or read off a percentage.

How to read it confidently:

• The largest slice (or tallest bar) is always nitrogen (≈ 78%).
• The next largest is oxygen (≈ 21%).
• A thin slice is argon (≈ 1%).
• Carbon dioxide (≈ 0.04%) is so small it is often shown as a barely-visible sliver, or labelled separately.

Identifying a gas from a percentage. If a question gives you a percentage, match it to the gas:

• ~78% → nitrogen
• ~21% → oxygen
• ~1% → argon (a noble gas)
• ~0.04% → carbon dioxide

A very common Paper 1 question gives a total volume of air and asks for the volume of one gas in it. The method is the same every time:

Step 1. Write the gas's proportion as a percentage (e.g. nitrogen = 78%).

Step 2. Turn the percentage into a decimal by dividing by 100 (78% → 0.78).

Step 3. Multiply by the total volume of air.

$\text{volume of gas} = \dfrac{\%}{100} \times \text{total volume of air}$

Worked example. Find the volume of nitrogen in 500 cm³ of dry air.

• Nitrogen ≈ 78%, so as a decimal that is 0.78.
• Volume of N₂ = 0.78 × 500 = 390 cm³.

You can work the other way too. If a sample of air has a volume of 200 cm³, the volume of oxygen is:

• Oxygen ≈ 21%, so 0.21 × 200 = 42 cm³.

Tip for the units. The proportions are by volume, so they work for any volume unit (cm³, dm³, m³) — just keep the total volume and the answer in the same unit.

A handy check. All four proportions should add up to (about) 100%: 78 + 21 + 1 + 0.04 ≈ 99.04% — the small shortfall is the trace gases. If your gas volumes add up to far more than the total volume of air, you have made an error.

Why ‘dry’? Real air always contains some water vapour, but the amount changes a lot — it can be anywhere from almost nothing in a desert to a few per cent on a humid day. Because it is not a fixed value, water vapour is left out of the standard figures, and we quote the composition of dry air instead. The proportions above are for the gases after the variable water vapour has been removed.

Argon — a noble gas. Argon makes up about 1% of dry air. It is one of the noble gases (Group 0), which means it is unreactive (inert). That is why it is used to fill electric light bulbs — it stops the hot filament reacting.

Carbon dioxide — tiny but important. CO₂ is only about 0.04% of the air, but it matters out of all proportion to its size:

• it is the raw material plants use in photosynthesis;
• it is a greenhouse gas, so changes in its small amount affect the climate.

Its level is slowly rising because of human activities such as burning fossil fuels — but for spec 2.6 you only need to recall the approximate value (~0.04%).