MCAT General Chemistry Question 52: Answer and Explanation

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Question: 52

7. In the reaction shown, if 39.05 g of Na₂S are reacted with 113.3 g of AgNO₃, how much of the excess reagent will be left over once the reaction has gone to completion?

Na₂S + 2 AgNO₃ → Ag₂S + 2 NaNO₃

A. 13.0 g Na₂S
B. 26.0 g Na₂S
C. 41.4 g AgNO₃
D. 74.3 g AgNO₃

Correct Answer: A

Explanation:

In this question, you are first given the masses of both reactants used to start the reaction. To figure out what will be left over, we must first determine which species is the limiting reagent. The formula weight of Na₂S is 78.1 grams per formula unit; the formula weight of AgNO₃ is 169.9 grams per formula unit. From this, we can determine that we are given 0.5 mol Na₂S and 0.67 mol AgNO₃. Because we need two moles of AgNO₃ for every mole of Na₂S, AgNO₃ is the limiting reagent, and the correct answer choice will be in grams of Na₂S. If 0.67 mol of AgNO₃ are used up, and Na₂S will be consumed at half the rate of AgNO₃ (based on their mole ratio), then 0.33 mol Na₂S will be used up. We then have 0.17 mol excess Na₂S, which has a mass of 13.0 g.

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