WAEC Chemistry 2019 Theory — Question 4
Question 4 of 28 from the West African Examinations Council (WAEC) Chemistry 2019 Theory paper, with the correct answer and a full explanation.
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1(d) Concentrated trioxonitrate(V) acid was added to a solution of iron(II) tetraoxosulphate(VI) and the mixture heated. The mixture turned from pale green to yellow with the evolution of a brown gas. Explain briefly these observations. [3 marks]
Model answer
The yellow colour obtained results from the oxidation of iron(II) ions (pale green, Fe²⁺) to iron(III) ions (yellow, Fe³⁺) by the concentrated HNO3. The brown gas evolved is nitrogen(IV) oxide (NO2), formed from the nitrogen(II) oxide produced in the reaction, upon exposure to air: 2NO + O2 → 2NO2.
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