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.