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.
Informacja do zadań 2a-2c.
Rhubarb leaves contain oxalate ions,
For each boiling time, three separate samples were prepared: 7.50 g of chopped leaves was added to 200 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.
(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.
(i) independent variable: boiling time AND dependent variable: volume/titre of ✔
(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 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:
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 and
suspended plant material/bubbles of make the solution cloudy and the colour change harder to see ✔
(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.
The dependent variable is what is measured directly in the laboratory, the titre; the amount of oxalate ions is calculated from it.
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