A sol is prepared by adding $20\mathrm{mL}$ of $0.10\mathrm{M}$ $\mathrm{KI}$ to $15\mathrm{mL}$ of $0.10\mathrm{M}$ $\mathrm{AgNO}{}_3$. Which row correctly gives the charge on the AgI particles and a coagulant that would be most effective for this sol?

Moles of $\mathrm{KI}$ = 2.0 mmol and of $\mathrm{AgNO}{}_3$ = 1.5 mmol, so $\mathrm{I}{}^{-}$ is in excess by 0.5 mmol. AgI adsorbs the common ion $\mathrm{I}{}^{-}$ and becomes negative. Coagulation is brought about by the cation, and the trivalent $\mathrm{Al}{}^{3+}$ is the most effective, whereas the anion of $\mathrm{K}{}_3[\mathrm{Fe}(\mathrm{CN}){}_6]$ has the same sign as the sol.