Question

A solution of `CoCI_(2).6H_(2)O` in isopropyl alcohol and water is purple. The color change to blue when we add
A. concentrated HCI
B. `AgNO_(3)(aq.)
C. both (1) and (2)
D. none of these

Answer 1

Correct Answer - A
The purple color is due to the mixture of `[Co(H_(2)O)_(6)]^(2+)` (pink) and `[CoCl_(4)]^(2-)` (blue):
`[Co(OH_(2))_(6)]^(2+)+4Cl^(-)hArr[CoCl_(4)]^(2-)+6H_(2)O`
When we add concentrated HCI, excess `Cl^(-)` shifts the equilibrium to the right (blue). Adding `AgNO_(3)` (aq.) removes some `Cl^(-)` by precipitation of `AgCl(s)` and favours the reaction to the left (produces more `[Co(OH_(2))_(6)]^(2+)` and the resulting solution is pink.