MCAT General Chemistry Question 58: Answer and Explanation
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Question: 58
13. In the following reaction:
Au2S3 (s) + H2 (g) → Au (s) + H2S (g)
If 2 moles of Au2S3 (s) is reacted with 5 moles of hydrogen gas, what is the limiting reagent?
- A. Au2S3 (s)
- B. H2 (g)
- C. Au (s)
- D. H2S (g)
Correct Answer: B
Explanation:
A limiting reagent is by definition a reactant. Because Au and H2S are products, they cannot act as limiting reagents, eliminating choices (C) and (D). Next, realize that what you are shown is an unbalanced equation. To answer this question, we must balance the reaction:
Au2S3 (s) + 3 H2 (g) → 2 Au (s) + 3 H2S (g)
We are given 2 moles of gold(III) sulfide and 5 moles of hydrogen gas. To use up both moles of gold(III) sulfide, we would need 6 moles of hydrogen gas because there is a 1:3 ratio between these reactants. We have only 5 moles of hydrogen gas, so that will have to be the limiting reagent.