WAEC Chemistry 2019 Theory — Question 20
Question 20 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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4(b) Consider the following equation: 2H2(g)+O2(g)→2H2O(g). Calculate the volume of unused oxygen gas when 40cm³ of hydrogen gas is sparked with 30cm³ of oxygen gas. [3 marks]
Model answer
Mole ratio of H2:O2 in the equation is 2:1, so 40 cm³ of H2 requires only 20 cm³ of O2 to react completely. Volume of unused (excess) oxygen = 30 − 20 = 10 cm³.
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