In the following decomposition reaction, identify p, q, r and s: p FeSO4 (s) → q Fe2O3 (s) + r SO2 (g) + s SO3 (g)
- A.3, 1, 1, 1
- B.1, 1, 1, 1
- C.1, 1, 2, 1
- D.2, 1, 1, 1
Correct answer
Explanation
The correct answer is D, 2, 1, 1, 1. The balanced equation is 2FeSO4 → Fe2O3 + SO2 + SO3, so p = 2, q = 1, r = 1 and s = 1. This is the thermal decomposition of ferrous sulphate. On strong heating the green crystals first lose their water of crystallisation and then break up into reddish-brown ferric oxide along with sulphur dioxide and sulphur trioxide gas. Check it by counting atoms: taking p = 2 gives two iron atoms on each side, two sulphur atoms on each side (one in SO2 and one in SO3) and eight oxygen atoms on each side, since the left has 2 into 4 and the right has 3 + 2 + 3. A is wrong because three iron atoms cannot fit into one Fe2O3. B is wrong because one FeSO4 supplies only one iron atom, while Fe2O3 needs two. C is wrong because it adds an extra SO2, leaving sulphur and oxygen unbalanced. Exam tip: balance iron first in 2FeSO4 → Fe2O3 + SO2 + SO3.
Practice Questions
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