Question

In: Chemistry

Gold is isolated from rocks by reaction with aqueous cyanide, CN-: 4 Au(s) + 8NaCN(aq)+O2(g)+H20(l)-> 4Na[Au(CN)2](aq)+4NaOH(aq)....

Gold is isolated from rocks by reaction with aqueous cyanide, CN-: 4 Au(s) + 8NaCN(aq)+O2(g)+H20(l)-> 4Na[Au(CN)2](aq)+4NaOH(aq). (a)Which atoms from which compounds are being oxidized, and which atoms from which compounds are being reduced? (b) The[Au(CN)2]- ion can be converted back to Au(0) by reaction with Zn(s) powder. Write a balanced chemical equation for this reaction. (c) How many liters of a 0.200 M sodium cyanide solution would be needed to react with 40.0kg of rocks that contain 2.00% by mass of gold?

Solutions

Expert Solution

(a)

The balanced chemical equation is as shown below:

The oxidation number of gold increases from 0 (in Au) to +1 (in ) . Hence, gold metal is oxidised. Au atoms are oxidised from gold metal. The oxidation number of oxygen decreases from 0 ( into ) to -2 (in ). Hence, oxygen atoms (from oxygen molecule) are reduced.

(b)

Zinc powder converts back into gold metal. The balanced chemical equation is as shown below:

(c)

Mass of rock

Mass percent of gold in the rock is 2.00%

Mass of gold

The atomic mass of gold is 197 g/mol.

Number of moles of gold

The molarity of NaCN solution is 0.200 mol/L

Volume of NaCN required

Hence, 40.6 L of 0.200 M NaCN solution are required.

queen_honey_blossom answered 2 months ago
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