Zadanie 3f
Wzorowane na: Chemistry HL Paper 1B, May 2026, TZ2, pytanie 3(f). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 3a-3f.
A titration was performed to find the concentration of iron(II) ions in a solution made from iron tablets. Portions of 25.00 cm3 of the iron(II) solution were acidified with excess dilute sulfuric acid and titrated with potassium manganate(VII) solution, which acts as its own indicator.
MnO4−(aq) + 5 Fe2+(aq) + 8 H+(aq) → Mn2+(aq) + 5 Fe3+(aq) + 4 H2O(l)
A standard 0.0200 mol dm−3 potassium manganate(VII) solution was used in a preliminary trial. The volume of titrant required was less than 5 cm3, so the standard solution was diluted before the main titrations.
Polecenie
Colorimetry is an alternative technique for determining concentration. Iron(II) ions can be converted to a red complex ion by adding 1,10-phenanthroline.
(i) Outline why the concentration of iron(II) in a solution can be measured by colorimetry after this reaction.
(ii) Predict a suitable wavelength to use in the colorimeter for a red solution. Use section 15 of the data booklet.
(iii) Sketch the calibration curve obtained by measuring the absorbance of solutions of the complex of known concentrations, with absorbance on the vertical axis and concentration on the horizontal axis.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
(i) the intensity of the colour/the absorbance of the complex depends on its concentration «because the complex absorbs visible light» ✔
(ii) any wavelength from 491 to 575 nm «a red solution absorbs green light» ✔
(iii) any straight line with a positive gradient «through or close to the origin» ✔
Schemat punktowania
(i) [1] for the link between the intensity of the colour (absorbance) and the concentration; mentioning the absorption of light is not required. (ii) [1] for a value or range inside 491 to 575 nm; accept 510 nm. (iii) [1] for a straight line with positive gradient and correctly labelled axes; a curve that levels off scores 0.
Komentarz
The colour absorbed is complementary to the colour seen, so on the colour wheel a red solution is measured with green light.