Zadanie 2a, Paper 1B, maj 2025, TZ2, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2025, TZ2, pytanie 2(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
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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(i) State the independent variable and the dependent variable of the investigation.
(ii) Apart from the mass of leaves and the volume of water, suggest two other conditions that should be kept constant for the comparison between boiling times to be valid.
(iii) Suggest a further trial that would allow the amount of oxalate ions that came only from the leaves to be found.
(iv) Discuss two problems that make the end point of this titration difficult to judge.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) independent variable: boiling time AND dependent variable: volume/titre of KMnO4(aq) ✔
(ii) Any two of:
the final volume of filtrate should be the same «made up with water after filtration» ✔
leaves from the same batch/of the same freshness/cut into pieces of the same size/with the stalk removed ✔
the water brought to the boil before the leaves are added ✔
the same time standing in the hot water after heating stops ✔
deionized/distilled water used instead of tap water ✔
the same storage time in the refrigerator ✔
the same temperature and the same volume of acid when titrating ✔
the KMnO4 solution kept in the dark/protected from light ✔
(iii) a sample of tap water boiled «for the same time» without any leaves and titrated in the same way ✔
(iv) Any two of:
KMnO4(aq) is intensely purple/opaque, so the meniscus is hard to see «and the burette has to be read from the top of the meniscus» ✔
the leaf extract is coloured/yellow-green, so the first permanent pale pink is masked/the end-point colour is a mixture of the two colours ✔
the reaction between MnO4− and C2O42− is slow «at first/at room temperature», so the colour fades slowly and the end point is easily overshot ✔
suspended plant material/bubbles of CO2 make the solution cloudy and the colour change harder to see ✔
Schemat punktowania
(i) [1] for both variables; 'amount of oxalate extracted' is not accepted as the dependent variable because it is derived from the titre. (ii) [2 max], list principle. (iii) [1] for a blank run with water but no leaves. (iv) [2 max], list principle; each difficulty must be linked to seeing the end point.
Komentarz. The dependent variable is what is measured directly in the laboratory, the titre; the amount of oxalate ions is calculated from it.