The coagulation values (mmol per litre) of a positively charged sol are $\mathrm{NaCl}$ 280, $\mathrm{CaCl}{}_2$ 270, $\mathrm{Na}{}_2\mathrm{SO}{}_4$ 2.4. Which of the following electrolytes should coagulate this sol at the lowest concentration?

The nearly equal values for $\mathrm{NaCl}$ and $\mathrm{CaCl}{}_2$ show that the cation valence has no effect, while $\mathrm{Na}{}_2\mathrm{SO}{}_4$ is far more effective than $\mathrm{NaCl}$. So the anion coagulates the sol, which is therefore positive. The highest-valent anion is $[\mathrm{Fe}(\mathrm{CN}){}_6]{}^{4-}$, so $\mathrm{K}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]$ is the best, while the cations of $\mathrm{AlCl}{}_3$, $\mathrm{CaCl}{}_2$ and $\mathrm{KBr}$ have no effective role.