Question

The electrolyte having maximum flocculation value for ${\text{AgI}}/{\text{A}}{{\text{g}}^ + }$ sol is:
A.${\text{N}}{{\text{a}}_2}{\text{S}}$
B.${\text{N}}{{\text{a}}_3}{\text{P}}{{\text{O}}_4}$
C.${\text{NaCl}}$
D.${\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4}$

Answer 1

Coagulation power is inversely proportional to flocculation value. Electrolyte with minimum coagulation power has maximum flocculation value or we can say electrolyte with maximum coagulation power has minimum flocculation value. Coagulation value depends on the valency of the ions.

Complete answer:
Coagulation is the process of converting the colloidal particles in the solution into the insoluble particles. It is brought about by adding an electrolyte to a colloidal solution. When the colloid coagulation or precipitation by adding suitable electrolyte is called flocculation.
According to Hardy Schulze rule, a positively charged colloid is coagulated by a negative ion and a negatively charged colloid is coagulated by a positive ion. The minimum amount of an electrolyte required to coagulate a colloid is called coagulation value of the colloid. Coagulation value is generally expressed in millimoles per litre. Lower the coagulation value, more efficient is the electrolyte in causing coagulation of sol. We can say coagulation value is inversely proportional flocculation value.
Increase in valence of an ion decreases coagulation value of the colloid. For example decreasing order of coagulating power of various cations are ${\text{A}}{{\text{l}}^{ + 3}} > {\text{M}}{{\text{g}}^{ + 2}} > {\text{N}}{{\text{a}}^ + }$ and for various anions are ${\text{PO}}_4^{ - 3} > {\text{SO}}_4^{ - 3} > {\text{C}}{{\text{l}}^ - }$ .
The coagulating powers of the electrolyte to coagulate the same colloidal solution can be calculated as:
$\dfrac{{{\text{coagulating power of A}}}}{{{\text{coagulating power of B}}}} = \dfrac{{{\text{coagulating value of B}}}}{{{\text{coagulating value of A}}}}$ .
${\text{N}}{{\text{a}}_2}{\text{S}} \to 2{\text{N}}{{\text{a}}^ + } + {{\text{S}}^{ - 2}}$
${\text{N}}{{\text{a}}_3}{\text{P}}{{\text{O}}_4} \to 3{\text{N}}{{\text{a}}^ + } + {\text{PO}}_4^{ - 3}$
${\text{NaCl}} \to {\text{N}}{{\text{a}}^ + } + {\text{C}}{{\text{l}}^ - }$
${\text{N}}{{\text{a}}_2}{\text{S}}{{\text{O}}_4} \to 2{\text{N}}{{\text{a}}^ + } + {\text{SO}}_4^{ - 2}$
As maximum flocculation value will be for the electrolyte having minimum coagulating power. Hence, ${\text{NaCl}}$ having lowest charge will have minimum power and hence have maximum flocculation value.

Thus, the correct option is C.

Note:
Electrolytes with less valence charge have less coagulation power and have greater coagulation value or flocculation value. As for electrolyte with less coagulation power, it requires more electrolyte to coagulate a colloid.