Question

When aqueous sodium hydroxide is added to an aqueous solution of chromium (III) ions, a green blue precipitate is first formed which re-dissolves to give a green solution. This green colour is due to:

• A. ${\left[Cr{\left(H_{2}O\right)}_6\right]}^{3+}$
• B. ${Cr}_4^{2-}$
• C. ${\left[Cr{\left(OH\right)}_4\right]}^{-}$
• D. $\left[Cr{\left(OH\right)}_3{\left(H_{2}O\right)}_3\right]$

Answer 1

Answer: (C)

${\left[Cr{\left(OH\right)}_4\right]}^{-}$

$\left[Cr\right(H_{2}O{)}_6{]}^{3+}+3O{H}^{-}\to \left[Cr\right(H_{2}O{)}_3\left(OH{)}_3\right]+3H_{2}O$ .
This will give green blue precipitate.
The precipitate will redissolve.
$\left[Cr\right(H_{2}O{)}_3\left(OH{)}_3\right]+O{H}^{-}\to {\left[Cr{\left(OH\right)}_4\right]}^{-}+3H_{2}O$. This will give green colour.