Metallic gold frequently is found in aluminosilicate rocks and it is finely dispersed among other minerals. It may be extracted by treating the crushed rock with aerated sodium cyanide solution. During this process metallic gold is slowly converted t `[Au(CN)_(2)]^(-)`, which is soluble in water. After equlibrium has been reached, the aqueous phase is pumped off and the metallic gold is recovered from it by reacting the gold complex with zinc, which is converted to `[Zn(CN)_(4)]^(2-)`, Gold in nature is frequently alloyed with silver is also oxidised by aerated sodium cyanide solution.
The correct ionic reaction for the process is//are:

Options

A. `Au+2CN^(-)rarrAu[(CN)_(2)]^(-)`
B. `Zn+2CN^(-)rarrZn[(CN)_(2)]^(-)`
C. `4Au+8CN^(-)+2H_(2)O+O_(2)("air")rarr4[Au(CN)_(2)]^(-)("soluble")+4OH^(-)`
D. `Zn+4CN^(-)rarrZn[(CN)_(4)]^(2-)`

Answer

C

Explanation

Oxygen containd in air oxidises `Au` to `Au^(+)` which then complexes with `CN^(-)` to form soluble complex.