Zadanie 1d, Paper 1B, maj 2025, TZ1, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2025, TZ1, pytanie 1(d). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
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A student studied how the concentration of sodium thiosulfate affects the rate of its reaction with dilute sulfuric acid.
Na2S2O3(aq) + H2SO4(aq) → Na2SO4(aq) + S(s) + SO2(g) + H2O(l)
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.
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The student's results are shown below. Each time was measured to ± 0.1 s.
| Concentration of Na2S2O3 / mol dm−3 | 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 cm3.
(i) Explain why the percentage uncertainty of the concentration becomes larger as the concentration decreases.
(ii) Estimate the rate of the reaction for the 0.2000 mol dm−3 solution, giving the correct units.
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Odpowiedź z klucza
(i) 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 ✔
(ii) «rate » 0.0610 ✔
s−1 ✔
Schemat punktowania
(i) [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. (ii) M1: 1/16.4 = 0.0610 (accept 0.061); M2: the unit s-1.
Komentarz. The end point fixes the amount of sulfur that has formed, so rate is taken as 1/t; the unit follows from that, not from mol dm-3 s-1.