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Aqueous CoCl2 is pink in colour at low concentrations but blue in high concentration

Low temperature are favourable for pink colour while high temperatures favourable blue colour

Addition of hydrochloric acid to pink solution of CoCl2 causes the solution to turn blue, again the pink colour is restored by the addition of HgCl2

Both pink and blue solutions are octahedral and paramagnetic complexes

answer is A.

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Detailed Solution

Octahedral CoH2O6Cl2 is pink and tetrahedral CoCl42− is blue. On high concentration tetrahedral CoCl42− is formed but in dilute solution it exists as pink CoH2O6Cl2. The formation of CoH2O6Cl2 is endothermic. So low temperatures are favourable. When HgCl2 is added to blue CoCl42−, it removes Cl−ions forming HgCl42− and CoH2O62+. Then pink color will be restored.

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