Which of the following pairs of cations can be separated by passing H2S through the mixture in the p — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: In Group III qualitative analysis, cations are precipitated as their hydroxides using NH4Cl and NH4OH. Step 2: The solubility product of ferrous hydroxide, Fe(OH)2 (Ksp ~ 8 x 10^-16), is much larger than that of ferric hydroxide, Fe(OH)3 (Ksp ~ 4 x 10^-38). Under the suppressed OH- conditions of the Group III buffer, Fe2+ may not precipitate completely. Step 3: Therefore, the solution is heated with conc. HNO3 to oxidize any Fe2+ to Fe3+ to ensure complete precipitation as Fe(OH)3, matching option (a).