Zadanie 2a
Wzorowane na: Chemistry HL Paper 3, May 2023, TZ2, pytanie 2(a). 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 student investigated how much spinach would supply the recommended daily intake of iron. The student
- weighed 63.2 g of spinach leaves and blended them with 600 cm3 of water
- boiled the mixture, filtered it and allowed the filtrate to cool
- pipetted 10.0 cm3 of the filtrate into 20.0 cm3 of 1.00 mol dm−3 sulfuric acid in a flask
- titrated the mixture with 0.00100 mol dm−3 potassium manganate(VII) solution.
The reaction taking place is:
5 Fe2+(aq) + MnO4−(aq) + 8 H+(aq) → 5 Fe3+(aq) + Mn2+(aq) + 4 H2O(l)
Polecenie
All the species in the titration are almost colourless except MnO4−, which is intensely purple. The spinach extract, however, is coloured green by chlorophyll.
(i) State the colour change at the end point.
(ii) Outline how adding distilled water to the 10.0 cm3 aliquot before the titration affects the titre at the end point.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) from green to purple ✔
OR
from green to brown ✔
OR
from green to purple-green ✔
(ii) none/no effect «the amount of Fe2+ in the aliquot is unchanged» ✔
Schemat punktowania
(i) [1]: green to purple, brown, purple-green or grey/bluish. A start colour given as "colourless" is tolerated, "clear" is not. "Purple to brown" and blue as the final colour are rejected. (ii) [1]: no effect; the reason is not required for the mark.
Komentarz
The mark scheme accepts any colour that a pale permanent excess of manganate(VII) can give against the green background.