which of the following electrolytes is most effective for the coagulation of Fe(OH)3 sol and why?

NaCl,Na2SO4,Na3PO4

Fe(OH)3 is positively charged sol. 
For coagulation wen need oppositely charged sol i.e. negatively charged sol.
Also, according to Hardy-Schulze  law which states : ‘the greater the valence of the flocculating ion, the greater will be its coagulating power’
Among NaCl, Na2SO4 and Na3PO4, Na3PO4 is most effective because of the greater charge (containing PO43-).Charge on NaCl and Na2SO4 is one and two respectively.Therefore, the correct option is 
Na3PO4.

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