Wzorowane na: Chemistry SL Paper 1B, May 2025, TZ1, pytanie 1(d). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Informacja do zadań 1a-1e.
A student studied how the concentration of sodium thiosulfate affects the rate of its reaction with dilute sulfuric acid.
The sulfur is produced as a fine suspension, so the mixture turns cloudy as the reaction proceeds. The student stood the beaker on a card with a black cross printed on it and recorded the time from mixing until the cross, viewed from above through the solution, could no longer be seen.
The student's results are shown below. Each time was measured to ± 0.1 s.
| Concentration of / | Time, trial 1 / s | Time, trial 2 / s | Time, trial 3 / s | Time, trial 4 / s | Time, trial 5 / s | Average time / s |
|---|---|---|---|---|---|---|
| 0.2000 ± 0.08 % | 17.2 | 15.9 | 16.8 | 15.6 | 16.5 | 16.4 ± 0.8 |
| 0.1000 ± 0.2 % | 33.6 | 32.1 | 34.0 | 32.8 | 33.3 | 33.2 ± 0.9 |
| 0.0500 ± 0.3 % | 65.4 | 67.9 | 66.2 | 64.8 | 67.1 | 66.3 ± 1.6 |
| 0.0250 ± 0.6 % | 131.7 | 129.4 | 133.0 | 130.2 | 132.4 | 131.3 ± 1.8 |
| 0.0125 ± 1.3 % | 261.8 | 265.0 | 259.6 | 263.9 | 262.7 | 262.6 ± 2.7 |
Rather than diluting the stock solution, the student prepared every solution separately, weighing the required mass of the solid on a balance reading to ± 0.01 g and making each one up to 250.0 .
Explain why the percentage uncertainty of the concentration becomes larger as the concentration decreases.
at lower concentration a smaller mass «of solid» is weighed out AND the absolute uncertainty of the balance «± 0.01 g» stays the same, so the relative/percentage uncertainty «0.01 g divided by a smaller mass» is larger ✔
[1]: the same absolute uncertainty on a smaller mass (or amount) gives a larger percentage uncertainty; accept an argument written as 0.01/m with m decreasing.
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