Zadanie 2c
Wzorowane na: Chemistry SL Paper 1B, May 2026, TZ3, pytanie 2(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 2a-2d.
A group of students analysed freshly squeezed pink grapefruit juice. They determined the content of the red pigment lycopene, a carotenoid, by colorimetry, and the content of ascorbic acid, C6H8O6, by titration.
Polecenie
The ascorbic acid in the juice was determined by titration with a standard solution of iodine, I2. The method is based on the following redox reaction:
C6H8O6(aq) + I2(aq) → C6H6O6(aq) + 2 I−(aq) + 2 H+(aq)
(i) Deduce the oxidizing agent in this reaction, giving the half-equation that shows its change.
(ii) Explain why starch solution is added near the end of the titration, and state what is observed when the end point is reached.
(iii) A 20.000 g (± 0.020 %) sample of juice was diluted with 50 cm3 of water in a conical flask and titrated against a standardized iodine solution of concentration 0.0100 mol dm−3 (± 0.50 %), delivered from a 50.00 cm3 burette (± 0.05 cm3). The mean titre was 3.10 cm3.
Calculate the percentage uncertainty in the volume of titrant delivered, showing your working.
(iv) Suggest one modification to the procedure that would lessen the effect of the largest source of uncertainty.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
(i) iodine / I2 «is the oxidizing agent» AND I2(aq) + 2 e− → 2 I−(aq) ✔
(ii) starch acts as an indicator: it gives a sharp colour change that helps the end point to be judged ✔
a blue-black colour appears when the first excess of iodine is present «all the ascorbic acid has reacted» ✔
(iii) «the titre is the difference of two burette readings, so the absolute uncertainty is 2 × 0.05 = 0.10 cm3»
«percentage uncertainty = =» 3.2 «%» ✔
(iv) Any one of:
use a less concentrated iodine solution «so that the titre is larger» ✔
OR use a larger mass of juice ✔
OR use a burette with a lower uncertainty ✔
Schemat punktowania
(i) [1] for the species AND the half-equation; do not award the mark for naming iodine without the half-equation. (ii) [2]: M1 starch acts as an indicator or sharpens the observation of the end point; M2 blue-black colour at the end point (accept 'colour change when the starch-iodine complex forms'). (iii) [1] for 3.2 %; award no mark for 1.6 %, which uses ± 0.05 cm3 once. (iv) [1] for one change; apply the list principle. A smaller burette scores only if it is linked to precision or uncertainty.
Komentarz
Of the three uncertainties quoted, 3.2 % for the titre is far larger than 0.50 % for the iodine solution and 0.020 % for the mass of juice, so the titre is the major source of uncertainty; a larger titre reduces its percentage uncertainty.