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

Arsenic sulphide (${\text{As}}_2{\text{S}}_3$) is a negatively charged (anionic) sol. According to the Hardy-Schulze rule:Coagulation of a negative sol is caused by cations.The coagulating power is directly proportional to the fourth power of the valency (charge) of the active ion.Charge order: ${\text{Na}}^{+}(+1)<{\text{Ba}}^{2+}(+2)<{\text{Al}}^{3+}(+3)$.Thus, the coagulating power order is: ${\text{Na}}^{+}<{\text{Ba}}^{2+}<{\text{Al}}^{3+}$.