According to Hardy-Schulze rule, coagulation power of ions is directly proportional to charge on ion. since Fe(OH)3 is positively charged colloid. ∴ It will be coagulated by anion. (a) Ma3(PO4)2 ⇄ 3Mg2++2PO43− (b) BaCl2 ⇄ Ba2++2Cl−1 (c) NaCl ⇄ Na++Cl− (d) KCN ⇄ K++CN− Because PO43− has highest charge among the given anions, therefore, Mg(PO4)−2 is the most effective in the coagulation of Fe(OH)3 solution