Zadanie 2d, Paper 1B, specimen 2025, Chemia IB SL
Wzorowane na: Chemistry SL Paper 1B, specimen 2025, pytanie 2(d). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
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.
PowiększPolecenie
The student's results are shown in the graph.
Powiększ(i) Calculate the average rate at which the iron(II) concentration fell in the 24 h flask during the 21 days. Include units.
(ii) Before starting, the student proposed the hypothesis: "The longer a solution is left open to the air each day, the less iron(II) it will contain, because oxygen in the air oxidizes iron(II)."
Discuss whether the results support this hypothesis.
(iii) State what the results imply about how the iron(II) solution should be stored so that it stays effective.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «(6.2 × 10⁻³ mol dm−3 − 1.4 × 10⁻³ mol dm−3) / 21 d =» 2.3 × 10⁻⁴ ✔
mol dm−3 d−1 ✔
(ii) «supports» longer daily exposure gives a lower iron(II) concentration
OR
the 0 h flask decreases the least ✔
«does not fully support» no direct evidence that oxygen is the oxidizing agent «another component of the air, or something else, could be responsible» ✔
(iii) store in a sealed/full container «with little air above the solution» ✔
OR
use within a few days of opening ✔
Schemat punktowania
(i) M1: value in the range 2.2 × 10⁻⁴ to 2.4 × 10⁻⁴; M2: units mol dm⁻³ d⁻¹. Accept values converted to other units, such as 2.5 × 10⁻⁹ to 2.8 × 10⁻⁹ mol dm⁻³ s⁻¹ or 0.22 to 0.24 mmol dm⁻³ d⁻¹. (ii) [1] for evidence from the data that supports the trend, [1] for recognizing that the data do not identify oxygen as the cause. (iii) [1] for one valid storage implication.
Komentarz. Read both concentrations from the 24 h series and divide by 21 d, not by the number of readings. In (ii) both sides of the discussion are needed for full marks.