MCAT General Chemistry Question 215: Answer and Explanation

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Question: 215

7. Given the following combustion reaction, calculate the mole fraction of hydrocarbon in the reactant solution before combustion. Assume neither starting material is limiting.

ZC_xH_y(g) + 8 O₂(g) → 5 CO₂(g) + 6 H₂O(l)

A. 1/8
B. 1/9
C. 2/9
D. 1/3

Correct Answer: B

Explanation:

B First, balance the equation:

C₅H₁₂(g) + 8 O₂(g) → 5 CO₂(g) + 6 H₂O(l)

The hydrocarbon must be C₅H₁₂ and the coefficient Z is 1. Since the question indicates that neither reactant is limiting, there must be a 1:8 molar ratio of hydrocarbon to oxygen present in the reaction flask, so the mole fraction (X) of hydrocarbon in the reactant solution before combustion is calculated by:

X = (moles hydrocarbon)/(total moles)

X = 1/(1+8) = 1/9

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