Question

The precipitates of $Fe(OH{)}_3$ and $Cr(OH{)}_3$ can be completely separated by :

• A. aqueous $N{H}_3$ (cold)
• B. $HCl$
• C. $NaOH/H_2{O}_2$
• D. $H_{2}S{O}_4$

Answer 1

The correct option is C $NaOH/H_2{O}_2$

$Fe(OH{)}_3$ and $Cr(OH{)}_3$ precipitates are completely seperated by $NaOH/H_2{O}_2$. Chromium hydroxide when dissolved in $NaOH/H_2{O}_2$, gets converted to chromium oxide giving a yellow coloured solution of $Cr{O}_4^{2-}$. While ferric hydroxide does not changes with $NaOH/H_2{O}_2$. It remains the same as brown precipitate.
$Cr(OH{)}_3+O{H}^{-}\rightleftharpoons [Cr(OH{)}_4{]}^{-}$
$2\left[Cr\right(OH{)}_4{]}^{-}+2O{H}^{-}+3H_2{O}_2\to 2Cr{O}_4^{2-}\left(\text{yellow coloured solution}\right)+8H_{2}O$
$Fe(OH{)}_3+NaOH/H_2{O}_2\to \text{no changes}$