Aqueous solution of a salt 'A', when mixed with NaOH solution and warmed, a black precipitate is for — Qualitative and Quantitative Analysis Chemistry Question
Question
Aqueous solution of a salt 'A', when mixed with NaOH solution and warmed, a black precipitate is formed. Black ppt. is filtered and dissolved in dil. H2SO4 solution. The resulting solution gives a chocolate coloured precipitate with potassium ferrocyanide solution. The filtrate obtained after filtering off the black precipitate, upon warming with Zn and NaOH evolves a pungent smelling gas. The resulting solution also responds to the ring test. The filtrate does not evolve any gas when it is boiled with urea in the presence of H2SO4.
💡 Solution & Explanation
Step 1: Separation of iron(III) hydroxide from aluminium hydroxide relies on their difference in amphoteric character. Step 2: Al(OH)3 is amphoteric and dissolves in excess sodium hydroxide (NaOH) to form soluble sodium aluminate (Na[Al(OH)4]), whereas Fe(OH)3 is purely basic and remains insoluble. Step 3: Therefore, adding NaOH solution successfully separates them by dissolving only Al(OH)3, leaving Fe(OH)3 as a precipitate, which matches option (c).