A reaction in an open flask gives off a gas. The flask stands on a balance, and the total loss in mass is recorded at different times.
| Time / s | Total loss in mass / g |
|---|---|
| 15 | 0.12 |
| 75 | 0.72 |
A student writes this calculation for the mean rate from 15 s to 75 s:
0.72 g−0.12 g75 s \frac{0.72\,\mathrm{g}-0.12\,\mathrm{g}}{75\,\mathrm{s}} 75s0.72g−0.12gWhich statement identifies the error in the student's calculation?
The numerator should be reversed to 0.12 g−0.72 g0.12\,\mathrm{g}-0.72\,\mathrm{g}0.12g−0.72g.
The numerator should be the final loss in mass, 0.72 g0.72\,\mathrm{g}0.72g, without subtraction.
The denominator should be the sum of the times, 75 s+15 s75\,\mathrm{s}+15\,\mathrm{s}75s+15s.
The denominator should be the elapsed time, 75 s−15 s75\,\mathrm{s}-15\,\mathrm{s}75s−15s.
122 exam-style questions on Edexcel GCSE Chemistry 8.1 Rates of reaction, covering 8.1.1 Measuring and interpreting rates of reaction, 8.1.2 Collision theory and the factors affecting rate, and 8.1.3 Catalysts and enzymes. Each one has a worked solution and a mark scheme showing where the marks go.