WAEC Chemistry 2021 Theory — Question 19
Question 19 of 31 from the West African Examinations Council (WAEC) Chemistry 2021 Theory paper, with the correct answer and a full explanation.
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3(b-ii). A burning magnesium ribbon was placed in a gas jar containing carbon (IV) oxide. (I) Write an equation for the reaction. (II) Explain briefly why the magnesium ribbon continues to burn in carbon (IV) oxide despite the gas not supporting combustion generally. (ii) Calculate the percentage by mass of nitrogen in magnesium trioxonitrate (V). [N=14.0, O=16.0, Mg=24.0]
Model answer
(I) 2Mg(s) + CO2(g) → 2MgO(s) + C(s). (II) The heat of reaction (enthalpy) released is high enough to decompose the CO2, releasing oxygen which then reacts further with the magnesium. (ii) Mg(NO3)2: molar mass = 24 + 2(14+3×16) = 24+124 = 148 g/mol. Mass of N = 28. %N = (28/148)×100 = 18.9%.
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