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Nessler's reagent  K2 Hg I4 can be formed from solutions of  Hg Cl2 + KI in water with K OH in water.
Nessler's reagent Potassium tetraIodo Mercurate K2 Hg I4 (II) can also be formed using solutions of  K I  in water, Hg Cl2 and then adding K OH and Hg Cl.

Thus Iodine atoms in the compound come from Potassium Iodide KI. 
   Since there are 4 Iodine atoms in one molecule of Nessler's ragent, we need 4 moles of KI for one mole of the reagent.

Hence,  0.4 * 4 = 1.6 moles of KI are required to produce 0.4 mole of K2 Hg I4.

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