Zadanie 5b
Wzorowane na: Chemistry SL Paper 2, May 2023, TZ1, pytanie 5(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 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
A 1.50 g sample of the double salt was dissolved in distilled water. Aqueous barium chloride was then added in excess, so that all the sulfate ions were precipitated as barium sulfate.
(i) Formulate an ionic equation, including state symbols, for the reaction of barium ions with sulfate ions.
(ii) Calculate the mass of barium sulfate precipitate. Use your answer to part 5a(iii) and section 7 of the data booklet. (If you did not obtain an answer to part 5a(iii), use 480.0 g mol−1 as Mr for the double salt, but this is not the correct value.)
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) Ba2+(aq) + SO42−(aq) → BaSO4(s) ✔
(ii) «n(salt) = 1.50 g/499.48 g mol−1 = 3.00 × 10−3 mol; two sulfate ions per formula unit, so» n(SO42−) = 6.006 × 10−3 mol ✔
«m(BaSO4) = 233.40 g mol−1 × 6.006 × 10−3 mol =» 1.40 g ✔
Alternative with the given 480.0 g mol−1: «n(SO42−) = 2 × 1.50/480.0 = 6.25 × 10−3 mol» m = 1.46 g
Schemat punktowania
(i) [1]: accept a single arrow or an equilibrium sign. (ii) [2]: award [2] for the correct final answer; accept the factor of 2 in any step; award [1] for half the answer (0.70 g), which comes from forgetting that there are two sulfate ions per formula unit.