Zadanie 2b
Wzorowane na: Chemistry SL Paper 1B, May 2026, TZ3, pytanie 2(b). 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
Before the samples were measured, an absorption spectrum was recorded to find the best wavelength to use.
(i) Describe the contents of the two solutions required to record the spectrum: the reference solution and the blank.
(ii) The spectrum obtained is shown.
Suggest the wavelength best suited to the determination of lycopene and give a reason for your choice.
(iii) The amount of lycopene was then found from a calibration curve. Suggest two conditions that the calibration curve must satisfy if it is to give reliable results.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
(i) Reference solution: lycopene of known concentration dissolved in heptane ✔
Blank: heptane / the solvent only ✔
(ii) «approximately» 470 «nm» ✔
this is the peak of greatest absorbance, where a small change in concentration gives the largest change in absorbance OR greatest sensitivity ✔
(iii) Any two of:
«at least» five different concentrations ✔
the range of concentrations includes the expected concentration of the sample ✔
precise data / concordant values for replicate samples ✔
strong correlation / R > 0.99 / points close to the line of best fit ✔
Schemat punktowania
(i) [2]: M1 reference solution containing lycopene of known concentration in the solvent; M2 blank containing the solvent only. Award ECF from 2a: if water was chosen there, accept lycopene in water for the reference solution and water for the blank; do not award M2 for heptane if ECF is being applied. (ii) [2]: M1 any value from 465 to 475 nm; M2 the wavelength of greatest absorbance, or the reason given in terms of greatest sensitivity. A wavelength of 444 nm or 502 nm does not score M1. (iii) [2 max]: one mark for each requirement; apply the list principle.