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When excess aqueous KI is added to HgCl2 solution, a precipitate X forms first, which then dissolves in excess KI to give a solution Y. Which of the following statements about X and Y is correct?

1. Y is a mixture of a black precipitate and a water-soluble complex
2. X is a green precipitate
3. The Hg atom in compound Y has sp³ hybridisation
4. A neutral solution of Y is used to detect NH3 gas

Correct answer: The Hg atom in compound Y has sp³ hybridisation

Solution

HgCl2 + 2KI gives HgI2 (X, scarlet-red precipitate, not green), which dissolves in excess KI to form K2[HgI4] (Y). In [HgI4]²-, Hg is tetra-coordinated in a tetrahedral geometry, giving sp³ hybridisation. Nessler's reagent (Y) is alkaline, not neutral, so option D is wrong.

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