Explain the following : i) Lead (Pb^(2+)) is placed in the first as well as second group of qualitative analysis. ii) The colour of mercurous chloride, Hg_(2),Cl_(2),changes from white to black when treated with ammonia. iii) During the qualitative analysis of a mixture containing Cu^(2+) and Zn^(2+) ions, H_(2)S gas is passed through an acidified solution containing these ions in order to test Cu^(2+) alone. Explain briefly.

Explain the following : i) Lead (Pb^(2+)) is placed in the first as w

1.	Explain the following : i) Lead (Pb^(2+)) is placed in the first as well as second group of qualitative analysis. ii) The colour of mercurous chloride, Hg_(2),Cl_(2),changes from white to black when treated with ammonia. iii) During the qualitative analysis of a mixture containing Cu^(2+) and Zn^(2+) ions, H_(2)S gas is passed through an acidified solution containing these ions in order to test Cu^(2+) alone. Explain briefly.
Answer» Solution :i) `PbCl_(2)` is partly soluble in WATER and hence `Pb^(2+)` ions pass to the first GROUP filtrate , i.e. to the II group . ii) Due to the formation of finely divided MERCURY. iii) `a) K_(sp)(CUS)` is less than `K_(sp)(CdS)`. b) Ionization of `H_(2)S` is further suppressed in presence of acid (common ION effect). `H_(2)S hArr 2H^(+)+S^(2-)` So, when `H_(2)S` gas is passed through acidified solution containing `Cu^(2+)` and `Zn^(2+)`, only `Cu^(2+)` ions will be precipitated due to low concentration of `S^(2-)` ions.