O-LevelChemistryAcids, Bases & Salts5070/21 · Oct/Nov 2017 · Q810 Marks

B8 Concentrated aqueous ammonia is used to make fertilisers such as ammonium phosphate, (NH4)3PO4-

(a)Calculate the percentage by mass of nitrogen in ammonium phosphate. % by mass[2]
(b)Explain why farmers spread nitrogen-containing fertilisers on their fields.[1]
(c)Describe a test for ammonia. test result[2]
(d)Explain why adding calcium hydroxide to the soil at the same time as ammonium phosphate results in loss of nitrogen from the soil.[2]
(e)Aqueous ammonia reacts with dilute sulfuric acid. 2NH3(aq) + H2SO4(aq) → (NH4)2SO4(aq) A student titrates 20.0cm³ of aqueous ammonia with 0.150mol/dm³ sulfuric acid. 10.5 cm³ of sulfuric acid is required to neutralise the aqueous ammonia. Calculate the concentration, in mol/dm³, of the aqueous ammonia. concentration of aqueous ammonia =[3]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[2] B8(a) 3×14 / 149 × 100 = 28.2%
[1] B8(a) If 2 marks not scored correct Mᵣ = 149
[1] B8(b) to increase plant growth / to help make more protein / to add nitrogen to soil depleted by previous year's growth
[1] B8(c) (damp) red litmus paper
[1] B8(c) turns blue
[1] B8(d) ammonia is formed
[1] B8(d) idea of ammonia escaping from the soil as a gas
[1] B8(e) mol H2SO4 = 0.150 × 10.5 / 1000 OR 1.575 × 10⁻³
[1] B8(e) mol NH3(aq) = 2 × 1.575 × 10⁻³ OR 3.15 × 10⁻³
[1] B8(e) concentration of NH3(aq) = 0.158 (mol dm⁻³)

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