O-LevelChemistryAcids, Bases & Salts5070/22 · Oct/Nov 2016 · Q1010 Marks

A student prepared some crystals of hydrated copper(II) sulfate by reacting excess insoluble copper(II) oxide with dilute sulfuric acid. CuO + H2SO4 + 4H2O → CuSO4.5H2O

(a)Describe how you would obtain pure dry crystals of hydrated copper(II) sulfate from the reaction mixture.[3]
(b)The student used 15.0 cm³ of 2.00 mol/dm³ sulfuric acid to prepare the crystals. Calculate the maximum mass of hydrated copper(II) sulfate crystals that could be made. g[3]
(c)Aqueous ammonia is added to aqueous copper(II) sulfate until the ammonia is in excess. What is observed as the aqueous ammonia is added?[2]
(d)An aqueous solution of copper(II) sulfate is electrolysed using inert electrodes. Predict the products of this electrolysis at the anode (positive electrode), the cathode (negative electrode).[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2016). The mark or mark type is shown in brackets.

[1] filter (off excess copper oxide)
[1] evaporate (filtrate) to crystallisation point/leave in warm place to crystallise/heat (solution) until saturated
[1] dry with filter paper/dry in a drying oven
[1] mol sulfuric acid = 2.0 × 15/1000 OR 0.03
[1] molar mass of hydrated copper sulfate = 250
[1] mass of hydrated copper sulfate = 7.5g
[1] (light) blue precipitate
[1] dark blue solution (in excess)
[1] anode: oxygen/O2
[1] cathode: copper/Cu

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