Question

On neutralizing a solution with with $N{H}_3$ solution and add $(N{H}_4{)}_{2}S$ in excess, the formation of white precipitate indicates presence of :

• A. $A{l}^{3+}$
• B. $Z{n}^{2+}$
• C. $C{r}^{3+}$
• D. Both a and b

Answer 1

The correct option is D Both a and b

$A{l}^{3+}$ and $Z{n}^{2+}$ belong to group 3 cations. Cations of this group do not react either with dilute hydrochloric acid, or with hydrogen sulphide in dilute mineral acid medium. However they form precipitates with ammonium sulphide in neutral or ammoniacal medium. A white precipitate may be caused by $A{l}^{3+}$ or $Z{n}^{2+}$. To a fresh sample add $N{H}_3$, first in moderate amounts, then in excess:
white precipitate, which dissolves in excess $N{H}_3$ solution $Z{n}^{2+}$ present
white precipitate, which remains unchanged if excess $N{H}_3$ is added: $A{l}^{3+}$ present