O-LevelChemistryStoichiometry5070/22 · Oct/Nov 2014 · Q39 Marks

Seawater contains a variety of dissolved salts. (a) The diagram shows a simple distillation apparatus that can be used to produce purified water from seawater. [Figure showing a simple distillation apparatus with labels: seawater, heat, water out, condenser, water in, purified water]

(a)Explain how distillation purifies seawater.[3]
(b)(i)Magnesium chloride, MgCl2, is present in seawater at a concentration of 1.26g/dm³. Write the formulae for the ions present in magnesium chloride.[1]
(b)(ii)Calculate the concentration of chloride ions, in mol/dm³, arising from the magnesium chloride in seawater. concentration = ................................. mol/dm³[1]
(b)(iii)Aqueous silver nitrate is added to a small sample of seawater. Describe what you would observe.[1]
(c)The concentration of sulfate ions in seawater is 1.24g/dm³. Excess aqueous barium chloride is added to a 50.0 cm³ sample of seawater. Calculate the mass of barium sulfate precipitated in this reaction. Ba2+(aq) + SO42-(aq) → BaSO4(s) mass = ................................. g[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[1] A3 (a) water and salts have different boiling points (1)
[1] A3 (a) water evaporates AND salts/residues/impurities/solids left in flask (1)
[1] A3 (a) water condenses/turns to liquid in the condenser (1)
[1] A3 (b) (i) Mg²⁺ and Cl⁻ (1) IGNORE: state symbols
[1] A3 (b) (ii) 0.0265/0.027/0.03 (mol/dm³) (1)
[1] A3 (b) (iii) white precipitate/white solid formed/white deposit formed (1)
[1] A3 (c) 96g SO₄²⁻ → 233g BaSO₄ (1)
[1] A3 (c) 1.24 g SO₄²⁻ → 233/96 × 1.24 OR 3.0096/3.01g BaSO₄ (1)
[1] A3 (c) mass in 50 cm³ = 3.01 × 50.0/1000 = 0.151 g (1) OR (for 1st two steps) moles SO₄²⁻ = 1.24/96 OR 0.0129 (1) mass of BaSO₄ = 0.0129 × 233 OR 3.01 g (1) OR mass of SO₄²⁻ in 50 cm³ = 1.24 × 50/1000 OR 0.062 g (1) moles SO₄²⁻ = 0.062/96 OR 0.000645833 mol (1) mass BaSO₄ = 0.000646 × 233 = 0.151g (1)

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