O-LevelChemistryAcids, Bases & Salts5070/21 · May/June 2019 · Q66 Marks

Propanoic acid is a weak acid. Calcium hydroxide and calcium oxide are bases.

(a)What is the meaning of the term acid in weak acid?[1]
(b)What is the meaning of the term weak in weak acid?[1]
(c)Describe how universal indicator can be used to find the pH of dilute propanoic acid.[1]
(d)Give a large scale use of calcium hydroxide that depends on its basic character.[1]
(e)Calcium oxide reacts with water to form calcium hydroxide. The reaction is exothermic. Use ideas about bond breaking and bond forming to explain why the reaction is exothermic.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] in aqueous solution contains hydrogen ions
[1] acid that does not dissocate completely / partial ionisation in water/ little dissociation (1)
[1] match the colour obtained with a colour chart (1)
[1] reducing acidity of soil / removing acidic gases from power station chimneys / flue gas desulfurisation (1)
[2] bond breaking is endothermic and bond making is exothermic (1)
[2] more energy released than absorbed (1)

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