Question

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

Answer 1

Hint- Arsenic sulphide $A{s_2}{S_3}$ is an anionic sol (negative sol) hence coagulation will depend upon coagulating power of cation, which is directly proportional to the valency of cation (Hardy schulze rule).

Complete answer:
We will apply the Hardy schulze law which says that-
The powerful particles of the electrolyte in achieving coagulation are those which convey charge inverse to that of the colloidal particles. These particles are called coagulating particles.
More prominent is the valency of the coagulating or the Flocculating particle, more noteworthy is its capacity to bring about the coagulation.
The coagulation is generally brought about by the addition of electrolytes.
The ion which is responsible for neutralization of charge on the colloidal particles is called the coagulating ion or Flocculating ion.
So, according to this rule, greater the charge on an oppositely charged ion, greater is its coagulation power. Since arsenic sulphide is a negatively charged sol, thus, the order of coagulating power is $N{a^ + } < B{a^{2 + }} < A{l^{3 + }}$.

Thus, option A is the correct option.

Note: There is another term known as the coagulation value which is defined by: it is the minimum of an electrolyte (in millimoles) that must be added to one litre of a colloidal solution so that it brings about the total coagulation, is called coagulation or Flocculation Value.