When a black metal sulphide reacts with dil. HCl, a gas liberates, which of the following pair of ca — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: The reaction of a black metal sulfide (like FeS) with dilute HCl releases hydrogen sulfide (H2S) gas: FeS + 2 HCl ---> FeCl2 + H2S↑. Step 2: In qualitative analysis, H2S gas is used to separate cations of different analytical groups. Under acidic conditions (dilute HCl), H2S selectively precipitates Group II cations (like Cd2+ as yellow CdS) due to their low solubility products, while Group IV cations (like Zn2+) remain soluble. Step 3: Therefore, passing the liberated H2S gas through an acidic mixture of Zn2+ and Cd2+ successfully separates them, corresponding to option (a).