Which of the following electrolytes is most effective in coagulating a negatively charged sol of ${\text{As}}_2{\text{S}}_3$?

Solution

1. The sol is negatively charged, so it requires a cation for coagulation.
2. Hardy-Schulze rule states that coagulating power increases with the charge of the effective ion.
3. The cations present are ${\text{Na}}^{+}$, ${\text{Ba}}^{2+}$, and ${\text{Al}}^{3+}$.
4. The charge on ${\text{Al}}^{3+}$ is the highest, making ${\text{AlCl}}_3$ the most effective.
5. Hence the answer is (C).