O-LevelChemistryAcids, Bases & Salts5070/21 · May/June 2017 · Q310 Marks

Acids are neutralised by alkalis.

(a)Write the ionic equation for the reaction between an acid and an alkali.[1]
(b)(i)Name a sodium compound and the acid that can be used to make sodium sulfate by this method.[1]
(b)(ii)Describe how the titration method is used to prepare a colourless solution of sodium sulfate.[3]
(b)(iii)Describe how a sample of pure sodium sulfate crystals can be made from aqueous sodium sulfate.[2]
(c)(i)Aqueous sodium sulfate can be used to prepare barium sulfate. Ba²+(aq) + SO₄²⁻(aq) → BaSO₄(s) In an experiment, 20.0 cm³ of 0.550mol/dm³ of barium nitrate was added to excess aqueous sodium sulfate. Calculate the maximum mass of barium sulfate that could be made. [The relative formula mass of BaSO₄ is 233.] maximum mass of barium sulfate = .............................................. g[2]
(c)(ii)A mass of 1.92g of dry barium sulfate was obtained. Calculate the percentage yield of barium sulfate. percentage yield of barium 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) H+ + OH- H2O (1)
A3(b)(i) Sulfuric acid AND sodium hydroxide (1)
A3(b)(ii) Place alkali in flask and acid in burette (1)
Add acid to alkali until indicator shows it is neutralised (1)
Repeat using same volumes but no indicator (1)
A3(b)(iii) Evaporate solution and allow to crystallise / (concentrate) by heating the solution until the first signs of crystallisation / heat to crystallisation point (1)
(Filter) wash with organic solvent / dry with filter paper / leave or dry in an oven (1)
A3(c)(i) Moles = 0.020 × 0.550 OR 0.011 (1)
Mass = 2.563 (1)
A3(c)(ii) Percentage yield = 74.91 (1)

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