MCAT General Chemistry Question 165: Answer and Explanation

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Question: 165

15. An assay is performed to determine the gold content in a supply of crushed ore. One method for pulling gold out of ore is to react it in a strong cyanide (CN^–) solution. The equation is provided below:

Au + NaCN + O₂ + H₂O → Na[Au(CN)₂] + NaOH

An indicator is used during this reaction, and approximately 100 mL of a 2 M NaCN solution is used to reach the endpoint. How many moles of Au are present in the crushed ore?

A. 0.01 mol
B. 0.02 mol
C. 0.10 mol
D. 0.20 mol

Correct Answer: C

Explanation:

First, balance the chemical equation:4 Au + 8 NaCN + O₂ + 2 H₂O → 4 Na[Au(CN)₂] + 4 NaOH

Now, determine the number of moles of NaCN used in the reaction:

If 0.2 mol NaCN are used in the reaction, then 0.2 mol

is oxidized.

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