Zadanie 2b, Paper 1B, specimen 2025, Chemia IB SL
Wzorowane na: Chemistry SL 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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A student investigates how contact with air affects the iron(II) concentration of an iron(II) sulfate solution prepared as a lawn treatment. The solution is divided into equal portions in identical stoppered flasks, which are kept in a dark cupboard at room temperature for three weeks. Every day the stopper of each flask is taken out for a set number of hours, different for each flask, as shown.
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The iron(II) concentration is found by titrating against a standard solution of potassium manganate(VII), KMnO4. At intervals of a few days, a 20.00 cm3 sample is withdrawn from each flask and made up to 250.0 cm3 with water; portions of this diluted solution are acidified and titrated.
(i) Suggest a reason for diluting the solution before the titration.
(ii) Identify a systematic error that this method could introduce for the flask whose exposure is meant to be zero hours.
(iii) Suggest how one extra flask could be prepared to test whether the error identified in (ii) actually occurs.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) Any one of:
to allow several titrations «from one sample» ✔
the solution is too concentrated «so the titre would be too large» ✔
end point «pink colour» easier to see «in a paler solution» ✔
(ii) the flask must be unstoppered every time a sample is taken «so its exposure is not truly 0 h» ✔
OR
there was air above the solution inside the flask from the start «so some oxygen reaches the iron(II) anyway» ✔
(iii) «set up a flask that is» opened and titrated only once, at the end of the three weeks ✔
OR
fill the space above the solution with nitrogen/argon/an inert gas ✔
OR
withdraw samples with a syringe through the stopper «without opening the flask» ✔
Schemat punktowania
[1] for each sub-part: (i) any one valid reason for dilution; (ii) a systematic error specific to the 0 h flask; (iii) a control flask that removes or isolates that error.