The amount of nickel(ii) iodide that should be added for making the solution is 71.3 kilograms.
As the molality and weight of water is given, we will use the formula of molality to find the mass of nickel(ii) iodide.
Molality = number of moles of nickel(ii) iodide/weight of solvent in kilograms
Number of moles = mass/molar mass
Molar mass = 312.5 grams
Keep the values -
0.643 = moles/355
Moles = 228.3
Mass = 228.3 × 312.5
Performing multiplication on Right Hand Side of the equation
Mass = 71,332.8 grams
Mass = 71.3 kilograms
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