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Question: Answered & Verified by Expert
An aqueous solution of $\mathrm{CoCl}_2$ on addition of excess of concentrated $\mathrm{HCl}$ turns blue due to formation of
ChemistryCoordination CompoundsAIIMSAIIMS 2005
Options:
  • A $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}_4 \mathrm{Cl}_2\right]\right.$
  • B $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}\right)_2 \mathrm{Cl}_4\right]^{2-}$
  • C $\left[\mathrm{CoCl}_4\right]^{2-}$
  • D $\left[\mathrm{Co}\left(\mathrm{H}_2 \mathrm{O}_2 \mathrm{Cl}_2\right]\right.$
Solution:
2463 Upvotes Verified Answer
The correct answer is: $\left[\mathrm{CoCl}_4\right]^{2-}$
$\mathrm{CoCl}_2$ is a weak Lewis acid, reacting with chloride ion to produce salt containing the tetrahedral $\left[\mathrm{CoCl}_4\right]^{2-}$ ion. $\mathrm{CoCl}_2$ is blue when anhydrous, and a deep magenta colour when hydrated, for this reason it is widely used as an indicator for water.

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