As per Hardy-Schulze formulation, the flocculation values of the following for ferric hydroxide sol are in the order :

Solution

Chemistry ma’am · 3 steps

Ferric hydroxide sol is positive, so look only at anions: higher anion charge, smaller flocculation value.

1. Charge on the sol. Fe(OH)₃ sol adsorbs Fe³⁺ ions, so its particles are positively charged. Only the oppositely charged ion of an added electrolyte, the anion, neutralises them and causes coagulation.
2. Hardy-Schulze rule. The greater the charge on the coagulating ion, the greater its coagulating power. Flocculation value is the minimum concentration of electrolyte needed to coagulate the sol, so it is inversely related to coagulating power. A higher anion charge gives a lower flocculation value.

   Electrolyte	Coagulating anion	Anion charge	Flocculation value
   K₃[Fe(CN)₆]	[Fe(CN)₆]³⁻	3	lowest
   K₂CrO₄	CrO₄²⁻	2	middle
   KBr	Br⁻	1	highest
   KNO₃	NO₃⁻	1	highest
   AlCl₃	Cl⁻ (not Al³⁺)	1	highest

3. So. K₃[Fe(CN)₆] < K₂CrO₄ < KBr = KNO₃ = AlCl₃. The three salts with a monovalent anion share the same flocculation value, so the answer is (b).

The idea to keep. First find the sign of the sol, then look only at the ion of opposite charge. Higher charge means stronger coagulation, and stronger coagulation means a smaller flocculation value.

Answer from the NTA final answer key. Solution by PrepWiser.