O-LevelChemistryAcids, Bases & Salts5070/21 · May/June 2015 · Q29 Marks

Some calcium compounds are used by farmers.

(a)Calcium hydroxide dissolves in water to form limewater. When carbon dioxide is bubbled through limewater, a white precipitate of calcium carbonate is formed. Construct the equation for this reaction.[1]
(b)Calcium hydroxide is used to neutralise acidic soils. Explain, using an ionic equation, why calcium hydroxide can neutralise acidic soils.[2]
(c)A farmer uses ammonium nitrate as a fertiliser on an acidic soil. He then uses calcium hydroxide to neutralise the acidic soil. Explain one disadvantage of using calcium hydroxide to neutralise this acidic soil.[2]
(d)The farmer uses another fertiliser. This fertiliser has the following percentage composition by mass. Са, 17.1%; H, 1.7%; P, 26.5%; Ο, 54.7% Calculate the empirical formula of this compound and suggest the formula of the anion present in the fertiliser. empirical formula is anion is[4]

Mark scheme

Marking points from the official mark scheme (May/June 2015). The mark or mark type is shown in brackets.

[1] (a) Ca(OH)2 + CO2 → CaCO3 + H2O (1)
[1] (b) Calcium hydroxide is a base/calcium hydroxide is an alkali/calcium hydroxide contains OH⁻ (1) H⁺ + OH⁻ → H2O (1)
[1] (c) Reacts (with ammonium nitrate) to give ammonia (1) Reduces nitrogen content of soil/ammonia escapes into the air (1)
[1] (d) Mole ratio line (1) Simplified ratio line (1) Empirical formula CaH4P2O8 (1) Anion H2PO4⁻ / H4P2O8²⁻ / PO4³⁻ (1)

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