Zadanie 2c
Wzorowane na: Chemistry HL 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. M(C6H8O6) = 176.14 g mol−1.
Calculate the percentage by mass of ascorbic acid in the juice.
(iv) Calculate the percentage uncertainty in the volume of titrant delivered, showing your working.
(v) The total percentage uncertainty in the result was found to be 3.7 %. Express the mass of ascorbic acid in 100 g of juice with its percentage uncertainty and with its absolute uncertainty. If you have no answer to part (iii), use 0.0250 %, which is not the correct value.
| Result from (iii) / g per 100 g | With percentage uncertainty | With absolute uncertainty |
|---|---|---|
(vi) 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) n(C6H8O6) = n(I2) = «3.10 × 10⁻³ × 0.0100 =» 3.10 × 10−5 «mol» ✔
«mass of ascorbic acid = 3.10 × 10⁻⁵ × 176.14 =» 5.46 × 10−3 «g» ✔
«percentage = =» 0.0273 «%» ✔
(iv) «the titre is the difference of two burette readings, so the absolute uncertainty is 2 × 0.05 = 0.10 cm3»
«percentage uncertainty = =» 3.2 «%» ✔
(v) With percentage uncertainty: 0.0273 ± 3.7 % «g per 100 g» ✔
With absolute uncertainty: 0.0273 ± 0.0010 «g per 100 g» ✔
(vi) 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) [3]: M1 amount of ascorbic acid equal to amount of iodine; M2 mass; M3 percentage. Award [3] for the correct final answer. (iv) [1] for 3.2 %; award no mark for 1.6 %, which uses ± 0.05 cm3 once. (v) [2]: M1 the result with the percentage uncertainty; M2 the result with the absolute uncertainty. Accept 0.027 ± 3.7 % and 0.027 ± 0.001 for M1 and M2. Award [1 max] if only the uncertainties are given without the original value. If 0.0250 % is used, accept 0.0250 ± 3.7 % and 0.0250 ± 0.0009. (vi) [1] for one change; apply the list principle. A smaller burette scores only if it is linked to precision or uncertainty.
Komentarz
The titre volume contributes 3.2 % of the 3.7 %, so it is the major source of uncertainty; a larger titre reduces its percentage uncertainty.