Question

Among the electrolytes $N{a}_{2}S{O}_4$, $CaC{l}_2$, $A{l}_2(S{O}_4{)}_3$, and $N{H}_{4}Cl$, the most effective coagulating agent for $S{b}_2{S}_3$ sol is:

• A. $N{a}_{2}S{O}_4$
• B. $CaC{l}_2$
• C. $A{l}_2(S{O}_4{)}_3$
• D. $N{H}_{4}Cl$

Answer 1

The correct option is C $A{l}_2(S{O}_4{)}_3$

• The process of settling of colloidal particles is called coagulation or precipitation of the sol.
  
• The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of a sol in two hours is called coagulating value.
  
• The greater the valence of the flocculating ion added, the greater is its power to cause precipitation. This is known as Hardy-Schulze rule.
• $S{b}_2{S}_3$ is a negative sol and hence a positive sol has to be used to coagulate it.
• Here, the valence of available species are $N{a}^{+},C{a}^{2+},A{l}^{3+},N{H}_4^{+}$. Thus, the maximum valence of $A{l}^{3+}$ will effectively coagulate the given $S{b}_2{S}_3$ sol.