The Earth’s Atmosphere

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

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

Nitrogen and oxygen, which exist as molecules (groups in sets of 2 atoms), make up 99% of the atmosphere. The other gases are found in smaller amounts.

Calculating the Percentage of Oxygen in the Air

This is how the percentage of oxygen has changed overtime:

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

1. Place pieces of copper into a test tube, on top of a Bunsen burner

2. Heat the pieces of copper strongly with the Bunsen burner

3. Syringes are used to pass air over the hot copper

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

Oxygen + CopperCopper oxide

or more specifically:

O2 + 2Cu2CuO

At the end, there should be no oxygen left in the air that was passed over the hot copper.

If you carried out a test, this is 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 ÷ Volume of air × 100

= 21 ÷ 100 × 100

= 21%