Which mixture of solutions will lead to the formation of negatively charged colloidal [AgI]I- sol?

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Question

Which mixture of solutions will lead to the formation of negatively charged colloidal [AgI]I- sol?

Accepted Answer

Correct Answer: C — 50 mL of 1 M AgNO3 + 50 mL of 2 M KISolution:When AgNO3 and KI are mixed, AgI precipitates. The charge on the colloidal AgI particle is determined by the ion that is present in excess (common ion adsorption).For a negatively charged sol ([AgI]I-), I- ions must be in excess, which adsorb on the AgI surface and impart a negative charge.Checking millimoles:A: mmol AgNO3 = 5, mmol KI = 5 &rightarrow; equal (no net charge, not a stable sol).B: mmol AgNO3 = 50, mmol KI = 75 &rightarrow; KI excess (I- excess) &rightarrow; negative sol (also correct in principle).C: mmol AgNO3 = 50, mmol KI = 100 &rightarrow; KI in large excess &rightarrow; I- excess &rightarrow; negatively charged sol.D: mmol AgNO3 = 100, mmol KI = 75 &rightarrow; AgNO3 excess (Ag+ excess) &rightarrow; positively charged sol.Option C has the largest and unambiguous excess of KI. (In the official NEET 2019 answer key, C is the correct answer.)Key Concept: The charge on a colloidal sol is determined by the ion present in excess (Paneth-Fajans rule). AgI sol with excess KI is negative ([AgI]I-); with excess AgNO3 it is positive ([AgI]Ag+).

Which mixture of solutions will lead to the formation of negatively charged colloidal [AgI]I- sol?

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Which mixture of solutions will lead to the formation of negatively charged colloidal [AgI]I- sol?

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