JAMB Chemistry 2019 Objective — Question 30

Question 30 of 40 from the Joint Admissions and Matriculation Board (JAMB) Chemistry 2019 Objective paper, with the correct answer and a full explanation.

20cm³ of a gaseous hydrocarbon and 150cm³ of oxygen were exploded in a closed vessel at room temperature. After cooling, 110cm³ of gases remained. After absorption by concentrated sodium hydroxide solution, the volume left was 50cm³. The molecular formula of the hydrocarbon is

A. C₃H₆
B. C₄H₆Correct
C. C₄H₁₀
D. C₄H₈

Explanation

Volume of CO₂ = 110−50 = 60cm³ (absorbed by NaOH); volume of O₂ used = 150−50 = 100cm³ (unused O₂). Using Avogadro's law with CₓHᵧ+(x+y/4)O₂→xCO₂+(y/2)H₂O and the given volumes (20cm³ hydrocarbon, 100cm³ O₂ used, 60cm³ CO₂): x=3 and y=8 from the ratio equations, giving C₃H₈. (Note: this derived formula, C₃H₈, does not correspond to any of the listed options; the source's stated key answer is option B, C₄H₆ — flagged as a possible data/option inconsistency in the source.)

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