Which one among the following pairs of ions cannot be separated by H2S in dilute HCl? — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Separation of cations using H2S in dilute HCl depends on Group classification. Group II cations (such as Hg2+, Pb2+, Bi3+, Cu2+, Cd2+, Sn2+, As3+, Sb3+) precipitate as sulfides in acidic media because of their extremely low solubility products (Ksp). Step 2: Group III and IV cations (such as Al3+, Zn2+, Ni2+, Co2+, Mn2+) do not precipitate in dilute HCl with H2S because their sulfides require a higher sulfide ion concentration (basic/ammoniacal medium) to precipitate. Step 3: Since both Bi3+ and Sn2+ belong to Group II, they both precipitate as sulfides (Bi2S3 and SnS) under acidic H2S conditions, meaning they cannot be separated from each other using this reagent. Thus, option (a) is the correct choice.