For coagulating As2S3 colloidal sol, which of the following will have the lowest coagulation value

The amount of electrolyte required to coagulate a fixed amount of a solution depends upon the valency of the flocculation ion. The flocculating power of the various ions follows the following order, larger the valency lesser will be coagulating value. Al3+>Mg2+>Na+, hence lowest coagulation value is of AlCl3.