Question

In quantitative analysis of second group in laboratory, \( H _{2} S \) gas is passed in acidic medium for ppt . When \( Cu ^{+2} \) and \( Cd ^{+2} \) react with KCN , then in which of the following condition, ppt will not be formed due to relative stability?

a. \( K _{2}\left[ Cu ( CN )_{4}\right] \) - More stable 

b. \( K _{2}\left[ Cd ( CN )_{4}\right] \) - Less stable 

c. \( K _{3}\left[ Cu ( CN )_{2}\right] \) - Less stable 

d. \( K _{3}\left[ Cd ( CN )_{3}\right]- \) More stable

Answer 1

(A) K₂[Cu(CN)₄] - More stable

Explanation:
In the presence of KCN, Cu²⁺ and Cd²⁺ form their respective cyano complexes. If the complex is more stable, the metal ion remains in solution, and precipitation (ppt) does not occur.

• K₂[Cu(CN)₄] is highly stable, so CuS precipitation does not occur.
• K₂[Cd(CN)₄] is less stable, allowing CdS precipitation.

Thus, ppt will not form when K₂[Cu(CN)₄] is formed.