Zadanie 13a
Wzorowane na: Chemistry SL Paper 3, November 2024, pytanie 13(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
Ethyl 4-hydroxybenzoate is a preservative used in some oral medicines. It was synthesized by heating 0.745 g of 4-hydroxybenzoic acid with excess ethanol and a few drops of concentrated sulfuric acid as catalyst.
C7H6O3 + C2H6O ⇌ C9H10O3 + H2O
Mr (4-hydroxybenzoic acid) = 138.13. Mr (ethyl 4-hydroxybenzoate) = 166.19.
(i) Calculate the theoretical yield of ethyl 4-hydroxybenzoate, in g.
(ii) Water was added to the cooled mixture and 0.99 g of solid product was isolated. Suggest, giving a reason, the identity of one possible impurity in the solid.
(iii) Suggest two differences between the melting behaviour of this impure solid and that of a pure sample of the ester.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «n = 0.745/138.13 = 5.394 × 10−3 mol»
«5.394 × 10−3 × 166.19 =» 0.896 «g» ✔
(ii) water AND not completely dried
OR
ethanol AND not all evaporated/removed
OR
4-hydroxybenzoic acid AND the reaction did not go to completion/it is a reversible reaction ✔ «the isolated mass exceeds the theoretical yield, so the solid cannot be pure»
(iii) melting starts at a lower temperature AND takes place over a wider temperature interval ✔
Schemat punktowania
(i) One mark for 0.896 g (0.90 accepted). (ii) One mark for a plausible impurity and a matching reason, e.g. water retained after incomplete drying. (iii) One mark only if both a lower melting point and a broader range are stated.
Komentarz
The isolated mass (0.99 g) is above the theoretical yield (0.896 g), which is the clue for (ii).