Question

What is the colour of the compound $CoC{l}_3.5N{H}_3$?

• A. Yellow
• B. Purple
• C. Green
• D. Violet

Answer 1

The correct option is B Purple

Cobalt(III) chloride forms coloured coordination compounds with ammonia.
Hexaamine cobalt chloride $\left(\right[Co\left(N{H}_3{)}_6{]}^{3+}3C{l}^{-}\right)$ is yellow in colour while the pentammine complex $\left(\right[CoCl\left(N{H}_3{)}_5{]}^{2+}2C{l}^{-}\right)$ is purple in colour. Thus option (b) is the correct answer.

Colour	Formula	Solution conductivity corresponds to
Yellow	$\left[Co\right(N{H}_3{)}_6{]}^{3+}3C{l}^{-}$	1 : 3 electrolyte
Purple	$\left[CoCl\right(N{H}_3{)}_5{]}^{2+}2C{l}^{-}$	1 : 2 electrolyte
Green	$\left[CoC{l}_2\right(N{H}_3{)}_4{]}^{+}C{l}^{-}$	1 : 1 electrolyte
Violet	$\left[CoC{l}_2\right(N{H}_3{)}_4{]}^{+}C{l}^{-}$	1 : 1 electrolyte

Atoms in square brackets do not dissociate.