Question

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

• A. addition of $NH_4OH$ in presence of $NH_4Cl$ when only $Fe(OH)_3$ is precipitated
• B. addition of $NH_4OH$ in presence of $NH_4Cl$ when $Cr(OH)_3$ and $Fe(OH)_3$ both are precipitated and on adding $Br_2$ 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 $Fe ^{3+}$ and $Cr ^{3+}$ both are present then very first solid ammonium chloride and ammonium hydroxide is added slowly till the solution smells of ammonia. $Fe ^{3+}$ and $Cr ^{3+}$ precipitates in the hydroxide form For identification precipitate is treated with $NaOH$ and $Br _{2}$ water, yellow colouration confirms $Cr ^{3+}$ ion.

$2 NaOH + Br _{2} \longrightarrow NaBrO + NaBr + H _{2} O$
$NaBrO \longrightarrow NaBr + O $

$2 Cr ( OH )_{3}+4 NaOH +3 O \longrightarrow $
$2 Na _{2} CrO _{4}+5 H _{2} O$

Solution is acidified and treated with lead acetate solution

$Na _{2} CrO _{4}+ Pb \left( CH _{3} COO \right)_{2} \longrightarrow \underset{\text{(yellow ppt.)}}{ PbCrO _{4}}$
$+2 CH _{3} COONa$