The composition of air is approximately
- 78% nitrogen (N2)
- 21% oxygen (O2)
- 1% argon (Ar)
- 0.03% carbon dioxide (CO2)
For IGCSE Chemistry, you need to know how to experimentally determine the percentage of oxygen in air!!
Method I
- Place some wet iron wool in the bottom of a test tube
- Invert the test tube in a beaker containing water
- Measure the height of the column of air in the test tube
- Leave the test tube for one week
- Measure the new height of the column of air
(a) Why is there a change in the height of column of air?
- Iron reacts with oxygen in the air to form iron oxide.
- Water level rises to fill up the volume of oxygen that has been used up.
- When all the oxygen has reacted, the height of the column of air will remain constant.
(b) How do you calculate the percentage of oxygen in this sample of air?
percentage of oxygen = volume of oxygen used up ÷ volume of initial amount of gas x 100
= change in height of column of air ÷ initial height of column of air x 100
= (80 - 63) ÷ 80 x 100
=21.25%
Method 2
The large pile of copper powder is heated, and the air in the syringe is passed several times from one gas syringe to the other. The large pile of copper powder turns black. The remaining gas is allowed to cool and its volume is measured.
(a) Explain why the large pile of copper turns black.
- The copper reacts with oxygen in the air to form copper oxide.
- The volume of gas changes with temperature.
- All the oxygen has reacted with the large pile of copper powder, so there is no oxygen left to react.