The coagulating power of electrolytes having ions ${\mathrm{Na}}^{+}$,  ${\mathrm{Al}}^{3+}$ and ${\mathrm{Ba}}^{2+}$ for arsenic sulphide sol increases in the order,

$\left[{\text{As}}_2{\text{S}}_3\right]{\text{ /S}}^{2-}$ is negatively charged sol.

According to the Hardy Schulze rule, the greater the charge on the cation (oxidation state), the greater is its coagulating power for the negatively charged sol. Hence, the increasing order is:

${\text{Na}}^{+}<{\text{Ba}}^{2+}<{\text{Al}}^{3+ }$

${\text{Al}}^{3+}$ (highest coagulating power).