O-LevelChemistryChemistry of the Environment5070/22 · Oct/Nov 2017 · Q810 Marks

Potassium nitrate, potassium sulfate and potassium phosphate are used in fertilisers.

(a)Calculate the percentage by mass of potassium in potassium sulfate, K2SO4· % by mass[2]
(b)Describe a test for sulfate ions. test result[2]
(c)Explain why nitrates in solid fertilisers spread onto soil are able to leach through the soil easily.[1]
(d)Nitrates are responsible for eutrophication. What is meant by the term eutrophication?[2]
(e)Dilute phosphoric acid, H3PO4(aq), reacts with aqueous potassium hydroxide to make potassium phosphate. H3PO4(aq) + 3KOH(aq) K3PO4(aq) + 3H2O(l) A student titrates 25.0cm³ of H3PO4(aq) with 0.200 mol/dm³ KOH(aq). 12.5 cm³ of KOH(aq) is required to react exactly with the H3PO4(aq). Calculate the concentration of the H3PO4(aq). concentration of H3PO4(aq) = mol/dm³[3]

Mark scheme

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

[2] 2×39/174 × 100 = 44.8% / 45%
[1] If 2 marks not scored correct Mr = 174
[1] (Acidified) barium chloride / barium nitrate
[1] White precipitate
[1] Nitrates soluble (in water) / nitrates dissolve (easily)
[1] (Nitrate causing) excessive growth of algae
[1] Bacterial growth (on dead algae)
[1] Idea of making water deoxygenated so animals / organisms can't live
[1] Mol KOH = 0.200 × 12.5/1000 OR 2.5 × 10⁻³
[1] Mol phosphoric acid = 2.5 x 10⁻³/3 OR 8.33 × 10⁻⁴
[1] Concentration of phosphoric acid = 0.0333 (mol / dm³) (8.33 × 10⁻⁴ × 1000/25)

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