9. 50 cm3 of Fe(OH)3 sol is poured into three flasks. To cause coagulation of the sol, it was necessary to add 5.30 cm3 of 1 M KCl to the first flask, 31.5 cm3 of 0.01 M Na2SO4 to the second, and 18.7 cm3 of 0.01 M Na3PO4 to the third. Calculate the coagulation thresholds of each electrolyte and determine the sign of the charge of the sol particles.

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