Edexcel iGCSE Chemistry -2.9 Composition of Dry Air- Study Notes- New Syllabus
Edexcel iGCSE Chemistry -2.9 Composition of Dry Air- Study Notes- New syllabus
Edexcel iGCSE Chemistry -2.9 Composition of Dry Air- Study Notes -Edexcel iGCSE Chemistry – per latest Syllabus.
Key Concepts:
2.9 know the approximate percentages by volume of the four most abundant gases in dry air
2.9 Approximate Percentages by Volume of the Four Most Abundant Gases in Dry Air
Dry air is a mixture of gases. The percentages given are by volume, which means they also represent the proportion of gas particles present.
The four most abundant gases in dry air are nitrogen, oxygen, argon and carbon dioxide.
Approximate Composition of Dry Air
| Gas | Chemical Formula | Approximate % by Volume |
|---|---|---|
| Nitrogen | \( \mathrm{N_2} \) | 78% |
| Oxygen | \( \mathrm{O_2} \) | 21% |
| Argon | \( \mathrm{Ar} \) | 0.9% |
| Carbon dioxide | \( \mathrm{CO_2} \) | 0.04% |
Important Notes
- The percentages are approximate.
- Water vapour is not included in dry air.
- Trace gases (e.g. neon, helium, methane) are present in very small amounts.
Total Percentage Check
\( 78 + 21 + 0.9 + 0.04 \approx 99.94\% \)
The remaining percentage is made up of other trace gases.
Example 1 (Conceptual):
Why is water vapour not included in the composition of dry air?
▶️ Answer/Explanation
Dry air means air with no water vapour.
Water vapour content varies depending on humidity.
Example 2 (Numerical):
If a room contains 500 dm³ of dry air, estimate the volume of oxygen present.
▶️ Answer/Explanation
Oxygen = 21%
\( \mathrm{Volume\ of\ O_2 = 0.21 \times 500 = 105\ dm^3} \)
Example 3 (Hard):
Explain why nitrogen is present in a much higher percentage than oxygen in the atmosphere.
▶️ Answer/Explanation
Nitrogen is relatively unreactive because of the strong triple bond in \( \mathrm{N_2} \).
It does not react easily with other substances.
Oxygen is more reactive and is used in processes such as respiration and combustion.
Therefore nitrogen accumulates in larger amounts in the atmosphere.