Analytical Chemistry — Question 6

(a) Ammonium hydroxide on reaction with lead salt solution gives chalky white precipitate of Pb(OH)₂. No precipitation occurs on adding Ammonium hydroxide to Calcium salt solution even when it is added in excess.

$\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2} ↓ + 2\text{NH}_4\text{NO}_3$

(b) When ammonium hydroxide solution is added to each of the compounds, lead nitrate forms a chalky white precipitate of lead hydroxide [Pb(OH)₂] which is insoluble in excess of ammonium hydroxide.

$\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2} ↓ + 2\text{NH}_4\text{NO}_3$

Whereas a gelatinous white precipitate of zinc hydroxide [Zn(OH)₂] is formed in case of zinc nitrate, which is soluble in excess of ammonium hydroxide.

Zn(NO₃)₂ + 2NH₄OH ⟶ 2NH₄NO₃ + Zn(OH)₂ ↓

(c) On adding Sodium hydroxide to Copper salt pale blue coloured precipitate is obtained which is insoluble in excess of Sodium hydroxide. Ferrous salt solution gives a dirty green coloured precipitate with Sodium hydroxide which is insoluble in excess of NaOH.

$\underset{\text{blue}}{{\text{CuSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}_2↓} + \underset{\text{colourless}}{\text{ Na}_2\text{SO}_4}$

$\underset{\text{pale green}}{{\text{FeSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{colourless}}{\text{Na}_2\text{SO}_4}$

(d) Sodium hydroxide on reaction with Fe(II) salt gives dirty green coloured precipitate, while with Fe(III) salt solution it forms reddish brown precipitate. Both precipitates are insoluble in excess NaOH.

Fe(II) salt:

$\underset{\text{pale green solution}}{{\text{FeSO}_4}} + 2\text{NaOH} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{colourless in solution}}{\text{Na}_2\text{SO}_4}$

Fe(III) salt :

$\underset{\text{yellow}}{{\text{FeCl}_3}} + \underset{\text{colourless}}{3\text{NaOH}} \longrightarrow \underset{\text{reddish brown ppt}}{\text{Fe(OH)}_3↓} + \underset{\text{colourless}}{3\text{NaCl}}$

(e) Ammonium hydroxide on reaction with lead nitrate gives a chalky white insoluble precipitate, and with ferrous nitrate forms a dirty green ppt.

$\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2↓} + 2\text{NH}_4\text{NO}_3$

$\underset{\text{colourless}}{{\text{Fe(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{ colourless}}{2\text{NH}_4\text{NO}_3}$