Question

Give a reason for the following:

For ferric hydroxide sol the coagulating power of phosphate ion is more than chloride ion.

Answer 1

This is because \[\ce{PO^{3-}4}\] ion has more charge than the Cl^– ion.

Answer 2

Ferric hydroxide is a positively charged sol; therefore, it will coagulate upon the addition of an anion. According to the Hardy-Schulze rule, the greater the valence of the flocculating ion (charge on ion) added, the greater is its power to cause precipitation or coagulation. Therefore, phosphate ion \[\ce{PO^{3-}4}\] being trivalent will coagulate the sol more readily than monovalent chloride ion Cl⁻.

Negative charge on \[\ce{PO^{3-}4}\] > Cl⁻

∴ Coagulating power of \[\ce{PO^{3-}4}\] > Cl⁻