Zadanie 5a
Wzorowane na: Chemistry SL Paper 2, May 2023, TZ1, pytanie 5(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 5a-5b.
Double salts contain two different cations and one anion. A hydrated sulfate of this type has the following percentage composition by mass.
| Element | Percentage by mass / % |
|---|---|
| Potassium (K) | 7.83 |
| Chromium (Cr) | 10.41 |
| Sulfur (S) | 12.84 |
| Hydrogen (H) | 4.85 |
| Oxygen (O) | to be calculated |
Polecenie
(i) Calculate the percentage of oxygen present in the double salt.
(ii) Determine the empirical formula of the double salt. Use section 7 of the data booklet.
(iii) The molar mass of the empirical formula is the same as the molar mass of the formula unit. Deduce the formula unit of the hydrated double salt.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «100 − (7.83 + 10.41 + 12.84 + 4.85) =» 64.07 % ✔
(ii) n(K) = 7.83/39.10, n(Cr) = 10.41/52.00, n(S) = 12.84/32.07, n(H) = 4.85/1.01, n(O) = 64.07/16.00 ✔
0.200, 0.200, 0.400, 4.80, 4.00 «mol per 100 g» ✔
dividing by 0.200 gives 1 : 1 : 2 : 24 : 20, so the empirical formula is KCrS2H24O20 ✔
(iii) KCr(SO4)2 ⋅ 12 H2O
OR KCrS2H24O20 «the empirical formula, since the molar masses are equal» ✔
Schemat punktowania
(i) [1]. (ii) [3]: M1 division by the relative atomic masses; M2 the amounts; M3 the whole-number ratio and formula. Award [3] for the correct final formula. (iii) [1]: accept KCrS2H24O20 (hydrate notation is not required by the 2023 guide), KCr(SO4)2(H2O)12 or the cations written in the other order.
Komentarz
The formula unit can be written with or without separating the water of crystallization; 12 of the 20 oxygen atoms belong to water.