The minimum concentration of an electrolyte required to cause coagulation of a sol is called its flocculation value. It is expressed in millimoles per litre. If the flocculation value standard As₂S₃ sol is 3.33. of ${\mathrm{MgSO}}_4$  How many milligrams of MgSO₄ is to be added to 20 ml standard As₂S₃ sol so that flocculation just starts ?