Analytical Chemistry — Question 2

(i) 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.

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

(ii) When ammonia solution is added dropwise to zinc sulphate, a white gelatinous ppt of zinc hydroxide is obtained.

$\underset{\text{colourless solution}}{{\text{ZnSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2↓} + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

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

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

(iii) When ammonia solution is added dropwise to iron (III) chloride, a reddish brown ppt. of Fe(OH)₃ is obtained.

$\underset{\text{yellow solution}}{{\text{FeCl}_3}} + 3\text{NH}_4\text{OH} \longrightarrow \underset{\text{reddish brown ppt}}{\text{Fe(OH)}_3↓} + \underset{\text{colourless in solution}}{3\text{NH}_4\text{Cl}}$

Excess of ammonia solution addition doesn't dissolve ppt.