Question bank › IGCSE Chemistry › Inorganic chemistry

Acids, bases and salt preparations

Answer on paper first, then reveal the mark scheme one mark at a time.

1Grade 6–76 marks

Describe how a student could prepare a pure, dry sample of copper(II) sulfate crystals, starting from copper(II) oxide and dilute sulfuric acid. [6]

warm the (dilute sulfuric) acid (in a beaker) (allow heat gently; reject boil)(1)
add copper(II) oxide (a little at a time) with stirring until it is in excess / no more dissolves / black solid remains(1)
filter (to remove excess copper(II) oxide)(1)
heat the filtrate to evaporate some of the water / until crystals start to form (accept heat to crystallisation point; reject evaporate to dryness)(1)
leave (the solution) to cool so that crystals form(1)
remove the crystals (e.g. filter) and pat dry with filter paper / dry in a warm place / leave to dry (reject heat strongly)(1)
Answer: warm acid, add excess CuO, filter, evaporate to crystallisation point, cool, dry crystals
Examiner tip: Evaporating to dryness destroys the hydrated crystals — say 'heat until crystals start to form' then leave to cool.

2Grade 6–7Triple only5 marks

Barium sulfate is an insoluble salt. A student mixes 20.0 cm³ of 0.500 mol/dm³ barium chloride solution with excess sodium sulfate solution.
$$\text{BaCl}_2(aq) + \text{Na}_2\text{SO}_4(aq) \rightarrow \text{BaSO}_4(s) + 2\text{NaCl}(aq)$$ [$M_r$ of BaSO₄ = 233]

(a) Calculate the maximum mass of barium sulfate that could form. [2]

(b) Describe how the student should obtain a pure, dry sample of barium sulfate from the mixture. [3]

(a) moles BaCl₂ = 0.500 × 20.0 ÷ 1000 = 0.0100 (mol) (= moles BaSO₄)(1)
(a) mass = 0.0100 × 233 = 2.33 (g) (correct answer with no working scores 2 marks)(1)
(b) filter (the mixture)(1)
(b) wash the residue / precipitate with (distilled) water(1)
(b) dry in a warm oven / leave to dry / pat dry with filter paper / dry in a desiccator(1)
Answer: (a) 2.33 g (b) filter, wash with distilled water, dry
Examiner tip: For an insoluble salt the product is the RESIDUE — don't evaporate the filtrate, that just gives sodium chloride.

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