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.