Question

A colourless aqueous solution contains nitrates of two metals X and Y when it was added to an aqueous solution of NaCl, a white precipitate was formed. This percipitate was found to be partly soluble in hot water to give a residue P and a solution Q. The residue P was soluble in a aq. NH_3 and also in excess sodium thiosulfate. the hot solution Q gave yellow precipitate with KI. The metals X and Y respectively are :
1) Ag and Pb
2) Ag and Cd
3) Cd and Pb
4) Cd and Zn ​

Answer 1

The metals X and Y respectively are Ag and Pb.

solution : case 1 : when nitrate of silver reacts with NaCl gives AgCl white precipitate.

chemical reaction is ...

AgNO₃ + NaCl ⇒AgCl (↓) + NaNO₃

AgCl precipitate dissolves in aqueous NH₃ and gives [Ag(NH₃)₂]⁺Cl¯

AgCl (↓) + NH₃ ⇒ [Ag(NH₃)₂]⁺Cl¯

also AgCl dissolves in thiosulfate (Na₂S₂O₃ ) and gives Na₃[Ag(S₂O₃)₂]..

AgCl (↓) + 2Na₂S₂O₃ ⇒Na₃[Ag(S₂O₃)₂] + NaCl

case 2 : same here Pb(NO₃)₂ reacts with sodium chloride and gives white precipitate and NaNO₃

Pb(NO₃)₂ + NaCl ⇒PbCl₂ (↓) + NaNO₃

and when hot solution of precipitate PbCl₂ is placed with KI , gives yellow precipitate PbI₂.

Therefore the correct metals are Ag and Pb. i.e., option (a) is correct choice