JEE Main · 2025 · 23 Jan 2025 · Shift 2 · Practical and Analytical Chemistry

This is the qualitative separation and confirmation sequence for lead. Treating the black precipitate of lead sulfide, $\mathrm{PbS}$ (A), with hot dilute $\mathrm{HNO}{}_3$ oxidises it to soluble $\mathrm{Pb}(\mathrm{NO}{}_3){}_2$; adding $\mathrm{H}{}_2\mathrm{SO}{}_4$ then throws down insoluble white $\mathrm{PbSO}{}_4$ (B). $\mathrm{PbSO}{}_4$ does not dissolve in dilute acids, but ammonium acetate solubilises it by forming a soluble lead–acetate species; acetic acid keeps this solution buffered without precipitating the lead prematurely. Adding $\mathrm{K}{}_2\mathrm{CrO}{}_4$ to this solution precipitates $\mathrm{PbCrO}{}_4$ (C), the characteristic yellow lead chromate used as a confirmatory test for $\mathrm{Pb}{}^{2+}$. The sequence $\mathrm{PbS}\to \mathrm{PbSO}{}_4\to \mathrm{PbCrO}{}_4$ identifies A, B and C.