O-LevelChemistryAcids, Bases & Salts5070/22 · May/June 2017 · Q310 Marks

Acids are neutralised by insoluble bases. (a) Magnesium chloride is a soluble salt that can be prepared from an insoluble base.

(a)(i)Name the acid and an insoluble base that can be used to make magnesium chloride.[1]
(a)(ii)Describe the experimental method used to prepare pure crystals of magnesium chloride from this acid and base.[4]
(b)Aqueous barium chloride and aqueous potassium sulfate can be used to prepare barium sulfate in a precipitation reaction. Write the ionic equation, including state symbols, for this reaction.[2]
(c)(i)Potassium sulfate can be prepared by reacting aqueous potassium hydroxide with dilute sulfuric acid. 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O In an experiment, 20.0 cm³ of 0.650 mol/dm³ sulfuric acid is just neutralised by aqueous potassium hydroxide. Calculate the maximum mass of potassium sulfate, K₂SO₄, that could be prepared. [The relative formula mass of K₂SO₄ is 174.][2]
(c)(ii)After crystallisation, 1.72 g of dry potassium sulfate was obtained. Calculate the percentage yield of potassium sulfate.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

A3(a)(i) Hydrochloric acid AND magnesium oxide
A3(a)(ii) 1 mark each for any 4 of: • Use of excess base (1) • Use hot acid / use warm acid / warm the mixture (of acid and base) (1) • Filter mixture (to get filtrate) (1) • Evaporate some of filtrate and allow to crystallise / leave in warm place to crystallise / heat to crystallisation point (1) • (Filter), wash with organic solvent/ dry with filter paper / dry in a (drying) oven (1)
A3(b) Ba²⁺(aq) + SO₄²¯(aq) → BaSO₄(s) Correct formulae and balancing (1) Correct state symbols – dependent on correct formulae (1)
A3(c)(i) Moles of acid = 0.020 × 0.65 OR 0.013 (1) Mass = 2.26(2) (g) / 2.3 (g) (1)
A3(c)(ii) Percentage yield = 76.(1) %

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