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  • 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. HNO_3, it forms a solution of blue colour.

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. HNO_{3}, it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution gives _________.

(i) a deep blue precipitate of Cu\left ( OH \right )_{2}

(ii) a deep blue solution of \left [ Cu\left ( N\! H_{3} \right )_{4} \right ]^{2+}

(iii) a deep blue solution of Cu\left ( NO_{3} \right )_{2}

(iv) a deep blue solution of Cu\left ( OH \right )_{2}. Cu\left ( NO_{3} \right )_{2}

Answers (1)

The answer is the option (ii). Black precipitate of copper sulphide is formed on passing H_{2}S through acidified solution of salt. On boiling with dilute HNO_{3} , the precipitate gives blue colour of copper nitrate. On addition of excess aqueous solution of ammonia, deep blue coloured compound is found. The reactions involved are:

Cu^{2+}+H_{2}S\rightarrow CuS (black)+2H^{+}

                                                                    CuS+dil.HNO_{3}\rightarrow Cu\left ( NO_{3} \right )_{2}

                                              Cu\left ( NO_{3} \right )_{2}+4NH_{3}\rightarrow \left [ Cu\left ( NH_{3} \right )_{4} \right ]^{2+}(Deep\; blue)

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