O-LevelChemistryStoichiometry5070/21 · Oct/Nov 2016 · Q510 Marks

Potassium chlorate, KClO3, decomposes to form potassium chloride and oxygen. 2KClO3(s) → 2KCl(s) + 3O2(g)

(a)Calculate the percentage by mass of oxygen in potassium chlorate.[2]
(b)Calculate the maximum volume of oxygen formed at room temperature and pressure when 12.25 g of potassium chlorate is completely decomposed.[3]
(c)(i)Potassium chloride can be made by reacting potassium with chlorine. Explain in terms of gain and loss of electrons, how potassium ions and chloride ions are formed when potassium reacts with chlorine.[3]
(c)(ii)Predict two physical properties of potassium chloride.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2016). The mark or mark type is shown in brackets.

[2] 39% (2) If 2 marks not scored, molar mass of KClO3 = 122.5 scores 1 mark
[1] KCIO3 = 12.25 / 122.5 OR 0.10 (mol) (1)
[1] moles O2 = 1.5×0.01 = 0.15 moles/idea of multiplying moles by 1.5 (1)
[1] volume of O2 = 3.6 dm³/3600 cm³, correct unit must be included (1)
[1] potassium (atom(s)) loses electron(s) (1)
[1] chlorine (atom(s)) gains electron(s) (1)
[1] so there are 8 electrons in outer shells of both ions (1)
[1] One mark each for any two of : • high melting point/high boiling point • dissolves in water • does not conduct electricity when solid/conducts electricity when molten

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