MCAT Chemistry Question — Chemical Reactions

by Allison Chae
Feb 12, 2017
Reviewed By: Liz Flagge

The combustion of glucose, a six-carbon sugar, is represented by the following balanced reaction:

C₆H₁₂O₆+ 6 O₂ → 6 CO₂ + 6 H₂O

If a reaction vessel initially contains 360 grams of C₆H₁₂O₆(MW = 180 g/mole), and if the reaction produces 440 grams of CO₂, what is the limiting reagent?

A. C₆H₁₂O₆

B. O₂

C. CO₂

D. H₂O

Click for Explanation

This question is asking the examinee to determine the limiting reagent of a chemical reaction. To solve this problem, you must calculate the theoretical amount of CO₂ that is produced given 360 grams of C₆H₁₂O₆. If all 360 grams (2 moles) of glucose initially present react, then 528 g (12 moles) of CO₂ should theoretically be produced.

360 g C₆H₁₂O₆x (1 mol C₆H₁₂O₆ / 180 g C₆H₁₂O₆) x (6 mol CO₂ / 1 mol C₆H₁₂O₆) x (44 g CO₂ / 1 mol CO₂) = 528 g CO₂

Since only 440 grams of carbon dioxide are actually produced, not all of the glucose combusts. Thus, the oxygen must be the limiting reagent in this problem, making answer choice B correct. Choices C and D can be eliminated because products are never limiting reagents. Since there will be leftover C₆H₁₂O₆, it is not the limiting reagent, eliminating answer choice A.

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