During an experiment to determine the percentage by volume of oxygen in air, a student passes a sample of air over heated copper turnings using a syringe system and a heated silica tube. The student records the following measurements:
| Measurement | Value |
|---|---|
| Volume of air in silica tube and connecting joints | 140 cm3140\text{ cm}^3140 cm3 |
| Syringe reading at start | 60 cm360\text{ cm}^360 cm3 |
| Syringe reading at end | 18 cm318\text{ cm}^318 cm3 |
| Volume of oxygen reacting | 42 cm342\text{ cm}^342 cm3 |
The student incorrectly calculates the percentage by volume of oxygen in air using this working:
42×10060=70% \frac{42 \times 100}{60} = 70\% 6042×100=70%Identify the mistake in the student's working.
Use the values in the table to correctly calculate the percentage by volume of oxygen in air.
