Zadanie 5a
Wzorowane na: Chemistry HL 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) Draw one Lewis (electron dot) structure of the sulfate ion.
(ii) Calculate the percentage of oxygen present in the double salt.
(iii) Determine the empirical formula of the double salt. Use section 7 of the data booklet.
(iv) 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) a Lewis structure of SO42− with S bonded to four O atoms and every atom shown with its valence electrons, for example four S-O single bonds with three lone pairs on each O and an overall charge of 2-
OR two S=O double bonds and two S-O single bonds with three lone pairs on each singly bonded O ✔
(ii) «100 − (7.83 + 10.41 + 12.84 + 4.85) =» 64.07 % ✔
(iii) 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 ✔
(iv) KCr(SO4)2 ⋅ 12 H2O
OR KCrS2H24O20 «the empirical formula, since the molar masses are equal» ✔
Schemat punktowania
(i) [1]: accept any combination of dots, crosses and lines; double bonds need not be opposite each other; do not penalize missing square brackets. (ii) [1]. (iii) [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. (iv) [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.