Question

The coagulation power of electrolytes having ions ${\text{N}}{{\text{a}}^ + }{\text{,A}}{{\text{l}}^{3 + }}{\text{ and B}}{{\text{a}}^{2 + }}$ for arsenic sulphide sol increases in the order.
A ) ${\text{B}}{{\text{a}}^{2 + }}{\text{ < N}}{{\text{a}}^ + } < {\text{A}}{{\text{l}}^{3 + }}$
B )${\text{A}}{{\text{l}}^{3 + }} < {\text{N}}{{\text{a}}^ + } < {\text{B}}{{\text{a}}^{2 + }}$
C )${\text{A}}{{\text{l}}^{3 + }} < {\text{B}}{{\text{a}}^{2 + }} < {\text{N}}{{\text{a}}^ + }$
D )${\text{N}}{{\text{a}}^ + } < {\text{B}}{{\text{a}}^{2 + }} < {\text{A}}{{\text{l}}^{3 + }}$

Answer 1

Hardy-Schulze rule states that greater is the charge on the ion of the electrolyte responsible for the coagulation, greater is its coagulating power. ${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$ sol particles carry a negative charge and are coagulated in presence of cations of the added electrolyte.

Complete answer:
Arsenic sulphide has a chemical formula of ${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$.
It is a negatively charged sol. It is an anionic sol. ${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$ sol particles carry a negative charge as they absorb anions. As long as ${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$ sol particles carry a negative charge, they will repel each other and do not coagulation. Thus, the charge on the sol particles helps in stabilizing the sols.
When an electrolyte is added to the ${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$ sol solution, the cations from the electrolyte will combine with the negative charges on ${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$ sol particles. Due to this, the charge on ${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$ sol particles gets neutralized. So these${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$ sol particles no longer repel each other. Thus, the coagulation of the ${\text{A}}{{\text{l}}_2}{{\text{S}}_3}$ sol can be achieved by adding an electrolyte.
Hardy-Schulze rule states that greater is the charge on the ion of the electrolyte responsible for the coagulation, greater is its coagulating power.
Thus, barium cation will have a greater coagulating power than sodium cation. This is because barium cation has greater charge than sodium cation. Similarly, aluminum cation will have a greater coagulating power than barium cation. This is because aluminum cation has greater charge than barium cation.

Hence, the correct option is the D )${\text{N}}{{\text{a}}^ + } < {\text{B}}{{\text{a}}^{2 + }} < {\text{A}}{{\text{l}}^{3 + }}$.

Note: The coagulating power of the electrolytes is expressed in terms of Gold number. When ${\text{1 ml}}$ of 10 % sodium chloride solution is added to ${\text{10 ml}}$ standard gold solution, the gold number represents the number of milligrams of protective colloid added to prevent the coagulation.