Zadanie 2b, Paper 1B, maj 2025, TZ2, Chemia IB SL
Wzorowane na: Chemistry SL Paper 1B, May 2025, TZ2, pytanie 2(b). 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.
PowiększPolecenie
(i) State what the graph shows about the relationship between the boiling time and the amount of oxalate ions extracted.
(ii) Comment on the range of boiling times and on the number of measurements made in the investigation.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) no relationship/the amount of oxalate ions extracted does not change «within uncertainty» as the boiling time increases from 4 to 16 minutes ✔
(ii) Range:
the range of boiling times is narrow/only 4 to 16 minutes
OR
all of the soluble oxalate has probably been extracted before 4 minutes, so this range cannot show a trend
OR
shorter «0 to 4 minutes» and/or longer boiling times are needed ✔
Number of measurements:
three repeats at each boiling time are enough to calculate a mean «and to judge the spread/random error»
OR
only four boiling times/a single interval between 0 and 4 minutes is not enough to establish a trend
OR
a measurement without boiling/at t = 0 is missing ✔
Schemat punktowania
(i) [1]; accept 'a slight decrease up to 12 minutes and then an increase' as a description of the plotted points. (ii) M1 for a comment on the range, M2 for a comment on the number of measurements; a comment may be positive or negative provided it is justified.