Composition of Air

IGCSE Edexcel Chemistry
2.9 Composition of dry air
Key Concept: Dry air is a mixture of gases with nitrogen and oxygen as the main components.

Section A: Percentages by Volume

1. State the approximate percentage by volume of nitrogen, oxygen, argon and carbon dioxide in dry air. [4]
2. Which gas makes up about 0.9% of dry air? [1]
3. Why are percentages given for dry air rather than moist air? [1]
4. A 100 cm3 sample of dry air contains 78 cm3 N2, 21 cm3 O2, 0.9 cm3 Ar and 0.04 cm3 CO2. Calculate the percentage of argon and carbon dioxide. [2]
5. Name one noble gas present in air. [1]

Total marks: 9

Mark Scheme

1. N2 about 78%, O2 about 21%, Ar about 0.9%, CO2 about 0.04% [4]
2. Argon [1]
3. Water vapor varies; dry air is consistent [1]
4. Argon 0.9%; CO2 0.04% [2]
5. Argon (or neon, helium, krypton, xenon) [1]