Question

Assertion in the coagulation of a negative sol the flocculating power is in the order:
$A{l^{3 + }} > B{a^{2 + }} > N{a^ + }$
Reason
Greater the valence of the flocculating ion added, greater is its power to cause precipitation.

Answer 1

A sol is colloid which is made of very small solid particles dispersed in continuous liquid medium. Blood is an example of sol. The minimum concentration of an electrolyte which is required to cause precipitation of sol is called the flocculating power of the ion of that electrolyte.

Complete step by step answer: Coagulation of sol a process in which colloidal particles are precipitated due to aggregation of the particles. Such phenomenon is the result of destabilization of colloids in order to neutralize the electric charge of the dispersed phase particles.

It is known that if an excess of an electrolyte is added to a solution or sol, the colloidal particles present in dispersed form are precipitated. This occurs due to the interaction between oppositely charged ions which is present in the colloid and the sol. As a result neutralization takes place and finally coagulation.
The ion which causes neutralization of charge on the colloid particles is referred to as the coagulating ion. Generally negatively charged ions cause precipitation of positively charged sol and vice versa.

In this aspect a rule was given by Hardy-Schulze which states that the coagulating powers of an electrolyte is directly proportional to the valency of the active ion. Hence, the more the valence of the active ion or flocculating ion added to a sol, the greater is its power to cause precipitation or coagulation.
For the given cations, the order of valence is$A{l^{3 + }} > B{a^{2 + }} > N{a^ + }$. So according to the Hardy-Schulze rule the greater will be the power to cause precipitation.

Note: In a similar way for positively charged sol the higher the negative charge of the added electrolyte the higher will be the degree of precipitation. For example the coagulating powers of anions like$P{O_4}^{3 - }$, $S{O_3}^{2 - }$ and $C{l^ - }$ are in the order$P{O_4}^{3 - } > S{O_3}^{2 - } > C{l^ - }$.