Analytical Chemistry — Question 8

(a) When sodium hydroxide solution is added to FeCl₃ dropwise, a reddish brown ppt is obtained, which is insoluble in excess of NaOH:

$\underset{\text{Yellow}}{{\text{FeCl}_3}} + \underset{\text{Colourless}}{3\text{NaOH}} \longrightarrow \underset{\text{Reddish brown ppt}}{\text{Fe(OH)}_3↓} + \underset{\text{Colourless}}{3\text{NaCl}}$

(b) When ammonia solution is added dropwise to cupper sulphate, a pale blue ppt of copper hydroxide is obtained.

$\underset{\text{Blue}}{{\text{CuSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{Pale Blue ppt}}{\text{Cu(OH)}_2} ↓ + \underset{\text{Colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

On adding excess of ammonia solution, the ppt dissolves and a deep blue solution is obtained.

$\text{Cu(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{Tetraammine copper (II) sulphate}}{[(\text{Cu(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$