To an aqueous solution containing ions such as Al3+, Zn2+, Ca2+, Fe3+, Ni2+, Ba2+ and Cu2+ was added conc. HCl,…

Step 1: Identify the operating analytical group The reagent specified is $\ce{H2S}$ gas in the presence of $\ce{HCl}$ (acidic medium). In qualitative inorganic analysis, this corresponds rigorously to the precipitating condition specifically distinguishing Group II cations.

Step 2: Understand the Common Ion Effect The high concentration of $\ce{H+}$ from the strong acid $\ce{HCl}$ strongly suppresses the ionization of the weak acid $\ce{H2S}$ via the common ion effect. $\ce{H2S <=> 2H+ + S^{2-}}$ This radically lowers the concentration of sulphide ions ($\ce{S^{2-}}$). Consequently, only the sulphides with extremely low solubility products ($K_{sp}$), which belong solely to Group II, will satisfy the ionic product to precipitate.

Step 3: Analyze each presented cation Let's evaluate the ions mapped to their traditional analytical groups:

• $\ce{Cu^{2+}}$: Group II (Precipitates as black $\ce{CuS}$)
• $\ce{Fe^{3+}}$: Group III (Reduced to $\ce{Fe^{2+}}$ by $\ce{H2S}$, but does not precipitate as sulphide in acidic medium)
• $\ce{Al^{3+}}$: Group III (Does not precipitate as sulphide)
• $\ce{Zn^{2+}}$: Group IV (Requires higher $\ce{S^{2-}}$ concentration; precipitates merely in alkaline medium)
• $\ce{Ni^{2+}}$: Group IV (Precipitates in alkaline medium)
• $\ce{Ca^{2+}}$, $\ce{Ba^{2+}}$: Group V (Do not form insoluble sulphides)

Step 4: Conclusion Only $\ce{Cu^{2+}}$ meets the specific condition to successfully precipitate as $\ce{CuS}$. Thus, only 1 cation precipitates.

Key Points to Remember:

• H2S in acidic medium (dil/conc $\ce{HCl}$) selectively targets Group II exclusively.
• Higher groups possess higher $K_{sp}$ values and mandatorily necessitate a basic medium (which removes $\ce{H+}$ to amplify $\ce{S^{2-}}$) to trigger precipitation.