Question

To an aqueous solution containing ions such as Al$^{3+}$, Zn$^{2+}$, Ca$^{2+}$, Fe$^{3+}$, Ni$^{2+}$, Ba$^{2+}$ and Cu$^{2+}$ was added conc. HCl, followed by H$_2$S.
The total number of cations precipitated during this reaction is/are :

• A. 3
• B. 2
• C. 1
• D. 4

Hint:

Remember the common ion effect in qualitative analysis. Adding a strong acid (HCl) to H$_2$S solution decreases the sulfide ion concentration, allowing for the selective precipitation of metal sulfides with very low solubility products (Group II cations).

Answer 1

The Correct Option is C

The question involves understanding the precipitation of cations using a chemical process. When concentrated hydrochloric acid (\(\text{HCl}\)) is added to the solution, and then hydrogen sulfide (\(\text{H}_2\text{S}\)) is passed through, the conditions are set for the selective precipitation of metal sulfides.

1. The cations listed are \(\text{Al}^{3+}\), \(\text{Zn}^{2+}\), \(\text{Ca}^{2+}\), \(\text{Fe}^{3+}\), \(\text{Ni}^{2+}\), \(\text{Ba}^{2+}\), and \(\text{Cu}^{2+}\). 
2. Group 2 cations of qualitative analysis precipitate as their sulfides in an acidic medium such as that provided by \(\text{HCl}\)and \(\text{H}_2\text{S}\). Cations like \(\text{Cu}^{2+}\)readily precipitate out as sulfides under these conditions, forming \(\text{CuS}\).
3. The remaining cations do not precipitate as sulfides in this medium:
   • \(\text{Al}^{3+}\)and \(\text{Zn}^{2+}\)form hydroxides in more basic conditions, not sulfides.
   • \(\text{Ca}^{2+}\)and \(\text{Ba}^{2+}\)are alkaline earth metals that require a different reagent to precipitate.
   • \(\text{Fe}^{3+}\)and \(\text{Ni}^{2+}\)do not precipitate sulfides in acidic conditions as readily as \(\text{Cu}^{2+}\).
4. Thus, under the conditions described, only \(\text{Cu}^{2+}\)precipitates as sulfide, leading to the precipitation of only one cation.

Conclusion: The total number of cations precipitated during this reaction is 1. Hence, the correct answer is 1.