Zadanie 2c, Paper 1B, maj 2025, TZ2, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2025, TZ2, pytanie 2(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
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Rhubarb leaves contain oxalate ions, C2O42−. Finely chopped rhubarb leaves were boiled in water for different lengths of time. After cooling, the water was analysed for oxalate ions by titration with standardized potassium manganate(VII) solution, KMnO4(aq), after acidification with dilute sulfuric acid.
2 MnO4−(aq) + 5 C2O42−(aq) + 16 H+(aq) → 2 Mn2+(aq) + 10 CO2(g) + 8 H2O(l)
For each boiling time, three separate samples were prepared: 7.50 g of chopped leaves was added to 200 cm3 of tap water in a beaker and the mixture was boiled for 4, 8, 12 or 16 minutes. After filtering off the leaves, each filtrate was transferred to a stoppered bottle and kept in a refrigerator until it could be titrated.
The mean amount of oxalate ions extracted at each boiling time is shown on the graph.
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The hypothesis being tested was that a longer boiling time would extract more oxalate ions from the leaves.
(i) Evaluate the hypothesis in the light of the results, giving a reason.
(ii) Suggest one way in which the investigation could be changed so that it tests this hypothesis properly.
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Odpowiedź z klucza
(i) not supported AND the amount extracted does not increase with boiling time «between 4 and 16 minutes» ✔
OR
not supported AND no sample was analysed without boiling, so it cannot be said whether boiling makes any difference ✔
(ii) Any one of:
use much shorter boiling times «for example every minute from 0 to 4 minutes» ✔
include an unboiled sample «leaves left in cold water for the same time» ✔
extend the investigation to much longer boiling times ✔
use a larger mass of leaves «so that differences in the titre are larger than the uncertainty» ✔
use a more sensitive method of analysis «for example ion chromatography» ✔
Schemat punktowania
(i) [1] for 'not supported' with a reason based on the data; 'supported' scores 0. (ii) [1], list principle, for any one change that addresses the range of times, the missing unboiled sample or the sensitivity of the method.