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

According to Hardy Schulze rule, greater the charge on cation, greater is its coagulating power for negatively charged solution. So, order of coagulating power ${\mathrm{Na}}^{+}<{\mathrm{Ba}}^{2+}<{\mathrm{Al}}^{3+}$