Question

A white amorphous powder A when heated gives a colourless gas B, which turns lime water milky and the residue C which is yellow when hot but white when cold. The residue C dissolves in dilute HCl and the resulting solution gives a white precipitate on addition of potassium ferrocyanide solution. A dissolves in dilute HCl with the evolution of a gas which is identical in all respects with B. The solution of A as obtained above gives a white precipitate D on addition of excess of NH₄OH and on passing H₂S. Another portion of this solution gives initially a white precipitate E on addition of NaOH solution, which dissolves on further addition of the base. Identify the compound A to E.

Answer 1

The given information is as follows. 

From part (a), we conclude that B is CO₂ as it turns lime water milky : 

and C is ZnO as it becomes yellow on heating and is white in cold. Hence, the salt A must be ZnCO₃. From part (b), it is confirmed that C is a salt of zinc (II) which dissolves in dilute HCl and white precipitate obtained after adding K₄[Fe(CN)₆ is due to Zn₂[Fe(CN)₆]. 

From part (c), it is again confirmed that A is ZnCO₃ as on adding dilute HCl, we get CO₂ and zinc (II) goes into solution. White precipitate is of ZnS which is precipitated in ammonical medium as its solubility product is not very low. White precipitate E is of Zn(OH)₂ which dissolves as zincate, in excess of NaOH. Hence the given information is explained as follows.