Question

Aluminium hydroxide is precipitated in Group ____.

• A. IV
• B. II (A)
• C. III
• D. II (B)

Answer 1

The correct option is C III

For the analysis of cations belonging to groups I-VI, the cations are precipitated from original solution by using the group reagents.
Group I reagent is dil.HCl.
Here, the group I cations like $P{b}^{2+}$ precipitate as chlorides.

If group-I is absent, add excess of water to the same test tube and warm the solution and pass $H_{2}S$ gas
for 1-2 minutes. Here, the group II cations like $P{b}^{2+}$, $C{u}^{2+},A{s}^{3+}$ precipitate as sulphides.

If group-II is absent, take original solution and add 2-3 drops of $conc.HN{O}_3$ to oxidise $F{e}^{2+}$ ions to $F{e}^{3+}$ ions.
Heat the solution for few minutes, after cooling add a small amount of solid $N{H}_{4}Cl$ and an excess of $N{H}_{4}OH$ solution till it smells ammonia.
Appearance of gelatinous white precipitate indicates the presence of $A{l}^{3+}$ ions. It precipitate as aluminium hydroxide ( gelatinous white precipitate).

Hence, option (c) is correct.