Question

If $F{e}^{3+}$ and $C{r}^{3+}$ both are present in group III of qualitative analysis, then distinction can be made by :

• A. addition of $N{H}_{4}OH$ in presence of $N{H}_{4}Cl$ when only $Fe(OH{)}_3$ is precipitated
• B. addition of $N{H}_{4}OH$ in presence of $N{H}_{4}Cl$ when $Cr(OH{)}_3$ and $Fe(OH{)}_3$ both are precipitated and on adding $B{r}_2$ water and NaOII, Cr(OH)3 dissolves
• C. precipitate of $Cr(OH{)}_3$ and $Fe(OH{)}_3$ as obtained in (b) are treated with conc. $HCl$ when only $Fe(OH{)}_3$ dissolves
• D. both (b) and (c)

Answer 1

Answer: (B)

If $F{e}^{3+}$ and $C{r}^{3+}$ both are present then very first solid ammonium chloride and ammonium hydroxide is added slowly till the solution smells of ammonia. $F{e}^{3+}$ and $C{r}^{3+}$ precipitates in the hydroxide form For identification precipitate is treated with $NaOH$ and $B{r}_2$ water, yellow colouration confirms $C{r}^{3+}$ ion.

$2NaOH+B{r}_2⟶NaBrO+NaBr+H_{2}O$
$NaBrO⟶NaBr+O$

$2Cr(OH{)}_3+4NaOH+3O⟶$
$2N{a}_{2}Cr{O}_4+5H_{2}O$

Solution is acidified and treated with lead acetate solution

$N{a}_{2}Cr{O}_4+Pb{\left(C{H}_{3}COO\right)}_2⟶\underset{\text{(yellow ppt.)}}{PbCr{O}_4}$
$+2C{H}_{3}COONa$