Assertion : In a mixture of Cd ( II ) and Cu ( II ) , Cd ²⁺ gets precipitated in presence of KCN by H ₂ ~S . Reason : The stability constant of [ Cu ( CN )₄ ]³⁻ is greater than [ Cd ( CN )₄ ]²⁻ .

Step-by-step solution

Cd ²⁺+2 CN ⁻ Cd ( CN )₂ Cd ( CN )₂ +2 CN ⁻ [ Cd ( CN )₄ ]²⁻ [ Cd ( CN )₄ ]²⁻ is colourless compound and not too stable. When hydrogen sulphide gas is added, cadmium sulphide is precipitated. [ Cd ( CN )₄ ]²⁻+ H ₂ ~S CdS +2 H ⁺+4 CN ⁻ But in case of Cu ²⁺ , Cu ²⁺+2 CN ⁻ Cu ( CN )₂ 2 Cu ( CN )₂ Quickly decomposes 2 CuCN +( CN )₂ CuCN +3 CN ⁻ [ Cu ( CN )₄ ]³⁻ This complex is so stable (i.e., [ Cu ⁺ ] is too low) that H ₂ ~S cannot precipitate Cu ( I ) sulphide ( Cu ₂ ~S )