Question

When concentrated $HCl$ is added to an aqueous solution of $CoCl _{2}$, its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction?

• A. $\left[ CoCl _{4}\right]^{2-}$
• B. $\left[ CoCl _{6}\right]^{3-}$
• C. $\left[ CoCl _{6}\right]^{4-}$
• D. $\left[ Co \left( H _{2} O \right)_{6}\right]^{2+}$

Answer 1

Answer: (A)

When concentrated $HCl$ is added in an aqueous solution of $CoCl _{2}$ then it forms $\left[ CoCl _{4}\right]^{2-}$ complex.
$\underset{\text{Pink colour complex}}{\left[ Co \left( H _{2} O \right)_{6}\right]^{2+}}+4 Cl ^{-} \longrightarrow \underset{\text{Blue colour complex}}{\left[ Co \left( Cl _{4}\right)\right]^{2-}}+6 H _{2} O$