Question

When aqueous NaOH 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. The green colour is due to

• A. $\left[\mathrm{Cr}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}$
• B. $\left[\mathrm{Cr}(\mathrm{OH})_4\right]^{-}$
• C. $\text{CrO}_{4}^{2 -}$
• D. $\left[\mathrm{Cr}(\mathrm{OH})_3\left(\mathrm{H}_2 \mathrm{O}\right)_3\right]$

Answer 1

Answer: (B)

When aqueous solution of NaOH is added to aqueous solution of Cr (III) ion it produces

Which further redissolves in aq. NaOH (in excess) to produce dark green solution due to formation of $\left(\left[\text{Cr} \left(\text{OH}\right)_{4}\right]\right)^{-}$