Oxygen and Oxides
| Gas | Amount in Air (%) |
| Nitrogen | 78.1 |
| Oxygen | 21.0 |
| Argon | 0.9 |
| Carbon Dioxide | 0.04 |
(2.15) Composition of Air
This is the approximate composition of air. Memorize it.
There are also very small amounts of noble gases in the air.
(2.16)Showing That Air Contains About 1/5 Oxygen
Using Copper
The apparatus originally contains 100cm3 of air. This is pushed backwards and forwards of the heated copper, which turns black as copper(II) oxide is formed. This uses up the oxygen. On cooling, around 79cm3 of gas is left in the syringes – 21% has been used up. Therefore, the air contains 21% of oxygen.
Using the Rusting of Iron
Iron rusts in damp air, using oxygen up as it does so. The experiment shows some damp iron wool in a test tube containing air. The tube is inverted in a beaker of water and the level of the water in the tube is marked by a rubber band. The tube is left for a week or so for the iron to use up the oxygen to make…guess…iron oxide!
The water level rises in the tube as the oxygen is used up, and the new level can be marked using a second rubber band. You can find the actual volumes of the gases at the end of the experiment by filling the tube with water to each of the rubber bands in turn, and pouring it into a measuring cylinder. If the original volume was, say, 15cm3, and the final volume was 12cm3, then the oxygen used up measures 3cm3.
The percentage of oxygen in air was 3/15 x 100 = 20%.
Burning Phosphorus
This can be done by putting a bell jar into a beaker filled with water. Phosphorus on an evaporating dish is placed onto the water (the jar has no bottom). It is then touched with a hot metal rod, which starts the reaction between phosphorus and oxygen. Phosphorus uses up the oxygen to form phosphorus oxide, lowering pressure in the jar and therefore, making water levels rise in the jar. The water should rise up by 20%.
Equation:
(2.17)Making Oxygen in the Lab
Oxygen is most easily made in the lab from hydrogen peroxide solution using manganese(IV) oxide as a catalyst. The reaction is known as the catalytic decomposition (splitting up using a catalyst) of hydrogen peroxide.
2H2O2(aq) -> 2H2O(l) + O2(g)