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The solubility products of NiS , ZnS , CdS and HgS are 4.7 10⁻⁵, 1.6 10⁻²⁴, 8 10⁻²⁷ and 4 10⁻⁵³ respectively. An aqueous solution contains Ni ²⁺, Zn ²⁺, Cd ²⁺ and Hg ²⁺ of equal concentration. H ₂ ~S gas was passed into this solution very slowly. The first and the last ions that precipitate as sulphides are respectively.

Options

  1. ANi ²⁺, Hg ²⁺
  2. BHg ²⁺, Cd ²⁺
  3. CZn ²⁺, Hg ²⁺
  4. DHg ²⁺, Ni ²⁺

Correct answer

D. Hg ²⁺, Ni ²⁺

Step-by-step solution

aligned & K_ sp of NiS =4.7 10⁻⁵ & K_ sp of ZnS =1.6 10⁻²⁴ & K_ sp of CdS =8 10⁻²⁷ & K_ sp of HgS =4 10⁻⁵³ aligned Since, the solubility products of NiS is highest and that of HgS is lowest, it implies that Hg ²⁺ will start precipitating quickly than any other salt on introduction of H ₂ ~S (sulphide ion source). On the same account Ni ²⁺ will precipitate last.

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