Question

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 ............ .
(a) deep blue precipitate of Cu(OH)₂
(b) deep blue solution of [Cu(NH₃)₄]²⁺
(c) deep blue solution of Cu(NO₃)₂
(d) deep blue solution of Cu(OH)₂. Cu(NO₃)₂

Answer 1

In qualitative analysis when H₂S is passed through an  aqueous solution of salt acidified with dilute HCl, a black  precipitate is obtained. On boiling the precipitate with  dilute HNO₃, it forms a solution of blue color. Addition of  excess of aqueous solution of ammonia to this solution  gives deep blue solution of [Cu(NH₃)₄]²⁺.

Explanation:

• When hydrogen sulphate passed to salts aqueous solution which acidifies the dilute hydrochloric acid.
• In copper sulphide, black precipitate is formed. When CuS is treated with dilute HNO3, copper nitrate is formed in blue color. It is on boiling. When aqueous solution of ammonia is added tetra amine copper (II) sulphate in deep blue color was obtained.
• It is qualitative analysis.
• Copper with hydrogen sulphide to form copper sulphide.
• Copper sulphide with dilute nitric acid to form copper nitrate.
• Copper nitrate with aqueous solution of ammonia to form tetra amine copper (II) sulphate.

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