Question 3(ii)
Distinguish between the following as directed.
(a) Calcium nitrate and lead nitrate using NaOH.
(b) Ferrous chloride and ferric chloride using NH4OH.
(a) When sodium hydroxide (NaOH) solution is added to each of the compounds, lead nitrate forms a chalky white precipitate of lead hydroxide [Pb(OH)2] which is soluble in excess of sodium hydroxide and readily soluble in acetic acid.
Pb(NO3)2 + 2NaOH ⟶ Pb(OH)2 ↓ + 2NaNO3
Pb(OH)2 + 2NaOH (excess) ⟶ Na2PbO2 + 2H2O
Whereas a milky white precipitate is formed in case of calcium nitrate which is sparingly soluble in excess of NaOH.
Ca(NO3)2 + 2NaOH ⟶ Ca(OH)2 ↓ + 2NaNO3
(b) On reaction with ammonium hydroxide (NH4OH), ferrous chloride forms a dirty green ppt whereas ferric chloride forms a reddish brown ppt. Hence, the two can be distinguished easily.
FeCl2 + 2NH4OH ⟶ Fe(OH)2 ↓ + 2NH4Cl
FeCl3 + 3NH4OH ⟶ Fe(OH)3 ↓ + 3NH4Cl