The Earth’s Atmosphere

The three main gases in the Earth’s atmosphere are nitrogen, oxygen and argon. Below is a table showing the abundance of these gases in the atmosphere. 

GasAbundance
Nitrogen (N2)78%
Oxygen (O2)21%
Argon0.9%
Carbon dioxide0.04%

Nitrogen and oxygen, which exist as molecules (made up of two atoms each), make up 99% of the atmosphere. The other gases are found in smaller amounts.

Calculating the Percentage of Oxygen in the Air

The diagram below shows how the percentage of oxygen has changed over time:

We can calculate the percentage of oxygen in the air by reacting it with copper:

1. Place pieces of copper into a test tube positioned over a Bunsen burner.

2. Strongly heat the pieces of copper using the Bunsen burner.

3. Use syringes to pass air over the hot copper.

The oxygen in the air reacts with the copper to form copper oxide:

Oxygen + CopperCopper oxide

or more specifically:

O2 + 2Cu2CuO

By the end of the experiment, it is expected that all the oxygen in the air that passed over the hot copper will have reacted.

If you were to carry out this experiment, here’s what you would typically find:

Starting volume of the air in cm³100
Ending volume of the air in cm³79
Oxygen Volume in cm³ (Volume of air at the start – Volume of air at the end)21

To calculate the percentage of oxygen, the equation is:

Volume of oxygen ÷ Total volume of air × 100

= (21 ÷ 100) × 100

= 21%

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