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Question:

A solution containing a group-IV cation gives a precipitate on passing H2S. A solution of this precipitate in dil.HCl produces a white precipitate with basic NaOH solution and a bluish-white precipitate with basic potassium ferrocyanide. The cation is: Ni2+, Co2+, Mn2+, Zn2+?

Zn2+

Ni2+

Co2+

Mn2+

Solution:

Zn2+ ions react with H2S to form a white precipitate of ZnS
Zn2+ + H2S → ZnS↓ + 2H+
ZnS and MnS are soluble in cold dil HCl while NiS and CoS are insoluble.
ZnS produces a white precipitate of zinc hydroxide with NaOH
ZnS + 2NaOH → Zn(OH)2↓ + Na2S
ZnS forms a bluish-white ppt with basic potassium ferrocyanide
2ZnS + K4[Fe(CN)6] → Zn2[Fe(CN)6]↓ (bluish white) + 2K2S