Question 4
When iron is left in damp air, rust forms on its surface.
Question 4(b)
A student uses this apparatus to find the approximate percentage by volume of oxygen in air.
This is the student's method.
- place a graduated glass tube in a beaker of water
- place some damp iron wool and a rubber bung in the top of the tube
- record the reading of the water level in the tube
- leave the apparatus for a few days
- record the reading of the water level again
The diagram shows the readings at the start and at the end of the experiment.
start
end
Question 4(b)(i)
Use the readings to complete the table, giving all values to the nearest 0.5 cm3.
| Volume / cm3 | |
|---|---|
| reading at start | 20.5 |
| reading at end | ................................ |
| volume of oxygen used | ................................ |
Question 4(b)(ii)
The student uses these results to calculate the percentage by volume of oxygen in air.
Suggest why her calculated value is lower than the expected value.
Question 4(c)
The student repeats the experiment using the same apparatus.
These are her results for the second experiment.
volume of air in tube at start = 80.0 cm3
reading at start = 20.0
reading at end = 35.5
Use the results to calculate the percentage by volume of oxygen in air.
percentage = ................................ %