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  4. In qualitative analysis, when H2S is passed through an aqueous solution of salt acidified with dil. HCl, a black precipitate is obtained. On boiling the precipitate with dil HNO3, it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution gives

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Q. In qualitative analysis, when $ {{H}_{2}}S $ is passed through an aqueous solution of salt acidified with dil. $ HCl, $ a black precipitate is obtained. On boiling the precipitate with dil $ HN{{O}_{3}}, $ it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution gives

ManipalManipal 2013The p-Block Elements - Part2

Solution:

$ \underset{Salt}{\mathop{CuS{{O}_{4}}}}\,+{{H}_{2}}S\xrightarrow[{}]{{}}\underset{black\,ppt}{\mathop{CuS}}\,+{{H}_{2}}S{{O}_{4}}; $ $ CuS+2HN{{O}_{3}}\xrightarrow[{}]{{}}\underset{blue\,solution}{\mathop{Cu{{(N{{O}_{3}})}_{2}}}}\,+{{H}_{2}}S; $ $ Cu{{(N{{O}_{3}})}_{2}}+4N{{H}_{3}}\xrightarrow[{}]{{}}\underset{deep\,blue\,solution}{\mathop{{{[Cu{{(N{{H}_{3}})}_{4}}]}^{2+}}}}\,+2NO_{3}^{-} $