Hydrated magnesium sulfate (Epsom salt) is
MgSO₄·xH₂O — each mole of MgSO₄ is combined with x moles of
water of crystallisation. Heating drives the water off as
vapour, leaving anhydrous MgSO₄. Weighing before and after tells you the mass of
water lost, and moles = mass ÷ Mr turns both masses into a mole ratio:
x = mol H₂O ÷ mol MgSO₄ = 7.
(b) Heating gently for only 1 minute
This paper heats for a fixed time (gently for about 1 minute, then strongly for 3 minutes). If the salt is
heated only gently for 1 minute, not all the water of crystallisation is driven off — the mass after heating is
greater than expected, so the mass and moles of water lost (and x) come out too small.
(c) The QA logic
Test 1: nitric acid, then silver nitrate: the green solution turns blue and a white precipitate
forms in the blue solution → chloride (AgCl).
Test 2: with excess ammonia, Cu²⁺ first gives a light blue precipitate that
dissolves to a dark blue solution (the filtrate). Fe³⁺ gives a
red-brown precipitate, insoluble in excess, left on the paper as a brown solid (the residue).
AgCl also dissolves in ammonia, so it doesn't confuse the residue.
Test 3: barium nitrate: no observable change → no sulfate present.
Test 4: excess sodium hydroxide precipitates both hydroxides — the residue is a green solid;
every cation is removed, so the filtrate is colourless. Because it holds both hydroxides, the Test 4
residue alone cannot identify either cation.
(c)(iii): either cation earns the mark — copper(II) from the dark blue filtrate in Test 2, or
iron(III) from the brown residue in Test 2 (insoluble in excess aqueous ammonia). The examiner report
gives B as CuCl₂·2H₂O + FeCl₃·6H₂O.
Examiner tips (from the real report)
Write "no observable change" when nothing happens — leaving a blank loses the mark.
Never describe a solution as "clear" — the word is "colourless".
Don't swap residue (stays on the paper) and filtrate (runs through).
Record masses to the precision of the balance (2 d.p.) — don't round.