A sample which may contain silver, lead or mercury ions or some combination of t

Question

A sample which may contain silver, lead or mercury ions or some combination of these metals, is treated with chloride ions, whereupon a white precipitate is formed. The solution is heated and the hot solution is filtered. When the filtrate is treated with chromate ion (CrO4-2) no precipitate is formed. The remaining white solid in the filter paper has ammonia water poured through it. This turns the precipitate in the filter paper black. The new filtrate is treated with hydrochloric acid (HCl), giving a white precipitate. What metal ion or ions are in the solution? Explain your reasoning.

Explanation / Answer

In the 1st step they will form AgCl , PbCl2 and Hg2Cl2 respectively. In the second step on titration with CrO4-2 , there is formation of PbCrO4 (yellow ppt) will takes place , rest while solution contains Ag and Hg. when this solution is treated with NH4OH leads to formation of black compound that is Hg+Hg(NH2)Cl. This solution also have Ag(NH3)2Cl which on further reaction with HCl gives AgCl as white ppt.

so finally in the solution Ag+ and Cl- including NH4+ and OH- ions with H2O is present.

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