Zadanie 2b, Paper 1B, specimen 2025, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, specimen 2025, pytanie 2(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
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Sulfur dioxide, SO2, is added as a preservative to many fruit cordials (concentrated fruit drinks that are diluted before drinking). In the acidic cordial it is present mainly as hydrogensulfite ions, HSO3−(aq), which are slowly oxidized to sulfate ions by oxygen from the air.
A student investigates how exposure to the air affects the hydrogensulfite concentration of one brand of blackcurrant cordial. The student fills identical flasks with equal volumes of the cordial, seals them and stores them in a refrigerator for 12 days. Once a day each flask is opened for a fixed period, which differs from flask to flask, as shown.
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The student measures the hydrogensulfite concentration by titrating samples against a standard iodine solution, with starch as the indicator. Every three days, 25.00 cm3 of cordial is withdrawn from each flask, diluted to 250.0 cm3 with distilled water, and portions of the diluted solution are titrated.
(i) Suggest one reason for diluting the cordial before the titration.
(ii) Identify a systematic error that this method could introduce for the flask labelled 0 h.
(iii) Suggest how the student could use one extra flask to check whether the error identified in (ii) has affected the results.
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Odpowiedź z klucza
(i) Any one of:
several titrations can be carried out from one small sample ✔
OR
the cordial is too concentrated «the titre would be too large / too much iodine solution would be needed» ✔
OR
the dark colour of the cordial is reduced, so the end point «blue-black starch-iodine colour» is easier to see ✔
(ii) the stopper must be removed every three days to take the sample «so even this flask is briefly exposed to the air and is not truly 0 h» ✔
OR
air «oxygen» is present above the cordial when the flask is sealed «so the hydrogensulfite is exposed to oxygen even in the 0 h flask» ✔
(iii) a flask that is sealed at the start and opened and titrated only once, at the end of the 12 days ✔
OR
a flask flushed with nitrogen/argon/an inert gas before it is sealed ✔
OR
samples withdrawn through the stopper with a syringe «without opening the flask» ✔
Schemat punktowania
(i) [1 max] for any one valid reason. (ii) [1] for identifying that the 0 h flask is nevertheless exposed to air (opening for sampling, or air in the headspace). (iii) [1] for a control flask that removes the exposure identified in (ii); the answer must match the error given in (ii).