Flocculation value is the number of millimoles of the electrolyte needed for 1 litre of the colloidal solution.
Assuming 1 litre of the solution,
mass of 1litre of 2 molal NaCl 1000×1.117 = 1117 g.
Moles of NaCl = 2
Mass of NaCl present in 1117 g of the solution =2×58.5=117 g
Mass of NaCl present 111.7 mg of the solution 117×0.11171117=0.0117g=11.7 mg
mmoles of NaCl = 11.758.5=0.2
0.2 mmole of NaCl required for 100 mL of the solution
Millimole of NaCl for 1 litre of the aresenious sulphide solution
=0.2100×1000=2