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The Atmosphere

The Proportion of Oxygen in the Air.

What is the Amount of Oxygen in the Air?

Oxygen makes up 21% of the air. This can be shown by the
reaction of a fixed amount of air with heated copper.
Copper will react with oxygen in the air to form copper(II) oxide.

copper   +   oxygen         copper(II) oxide.
2Cu_(s)  +    O_2(g)               2CuO_(s)    

A known amount of air is passed backward and forward
over the heated copper.
As copper(II) oxide is formed, oxygen is removed from the air
and the volume of air in the gas syringes is seen to decrease.

When the reaction is complete, only 80% of
the original volume of air remains.
This shows that the proportion of oxygen in the air is about 20%.

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