2.3.2Inorganic Chemistry
Practical: Determine the % of Oxygen in Air — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 7 of 52
Part 1 of 3Method
1
Use a gas burette with the tap closed. Work out the volume of the empty space between the last mark on the scale and the tap, for example by filling it with water and draining it into a small measuring cylinder.
2
Moisten the inside wall of the burette with a little water, since iron needs both water and oxygen to rust.
3
Place a plug of iron wool or a few iron filings in the closed end so that it sticks to the damp glass.
4
Invert the burette into a trough of water and clamp it vertically, making sure the tap is closed so that the air cannot escape.
5
Record the position of the water level on the burette scale at the start.
6
Leave the apparatus for three to four days and then record the new water level, ensuring the level inside matches the level outside so that the pressure is atmospheric.
Part 2 of 3Worked calculation
| Step | Working | Result |
|---|---|---|
| Volume of the space below the scale | Measured separately | 3.8 cm3 |
| Initial volume of air | (50.0 + 3.8) − 2.6 | 51.2 cm3 |
| Final volume of air | 53.8 − 12.7 | 41.1 cm3 |
| Volume of oxygen used | 51.2 − 41.1 | 10.1 cm3 |
| Percentage of oxygen | 10.1 ÷ 51.2 × 100 | 19.7%, close to 20% |
Part 3 of 3Variables, safety and reliability
Controlling the investigation
Variables
- Variables The independent variable is the time left; the dependent variable is the volume of gas remaining.
- Control the temperature, use the same starting volume of air and keep the amount of moisture constant.
Safety
- Safety Wear eye protection and clamp the glassware securely so it cannot fall.
- Wash hands after handling iron wool.
Improving the result
- Improving the result Repeat the experiment and calculate a mean.
- Make sure all the oxygen has reacted by checking that the water level has stopped rising.
- Use a large surface area of iron so the reaction goes to completion.
Key terms
| Term | Meaning |
|---|---|
| Oxidation | A reaction in which a substance gains oxygen, such as iron forming rust. |
| Burette | A long graduated glass tube with a tap, used here to hold and measure a volume of gas. |
| Trough | A container of water in which gas apparatus can be inverted. |
| Percentage by volume | The volume of one gas divided by the total gas volume, multiplied by 100. |
| Control variable | A quantity kept the same so that it cannot affect the result. |
Avoid these mistakes
1
Forgetting to add the volume between the 50 mark and the tap to the scale reading when finding the total air volume.
2
Dividing by the final volume instead of the initial volume of air when calculating the percentage.
3
Not dampening the iron. Without water the iron would hardly rust and the result would be too low.
4
Reading the water level when the levels inside and outside the burette differ, which means the gas is not at atmospheric pressure.
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