Question

For the coagulation of 500 mL of arsenious sulphide sol, 2 mL of 1M NaCl is required. What is the flocculation value of NaCl?
a.) 3
b.) 2
c.) 5
d.) 4

Answer 1

Flocculation, also known as coagulation is the process of precipitation of a colloidal solution by the addition of the excess of an electrolyte. Use the data given in the question to calculate the flocculation value for 1 L sol.

Complete step by step solution:

Let us solve this question by defining flocculating values. “The minimum concentration of an electrolyte that is required to cause the coagulation or flocculation of a sol is known as flocculation value.”
It is expressed in terms of millimoles per litre.
According to the question, 2 mL of 1M NaCl coagulates 500 mL of arsenious sulphide sol. Therefore, we can calculate the flocculating value for a sol as –
2 mL of 1 M NaCl contains 2 / 1000 mol = 2 millimole.
Therefore, 500 mL of arsenious sulphide sol requires 2 millimoles of NaCl.
If 500 mL of arsenious sulphide sol requires 2 millimoles of NaCl.
So, 1000 mL of arsenious sulphide sol requires 2x2 = 4 millimoles of NaCl.
Therefore, flocculation value of NaCl = 4.
Therefore, the answer is – option (d).

Additional Information: Flocculating value is inversely proportional to coagulating power.

Note: Coagulation is the process of precipitation of a colloidal solution on addition of an electrolyte. The coagulation capacity of any electrolyte depends on the valence of ion responsible for coagulation. The greater the valence of the oppositely charged ion of the electrolyte added to a colloidal solution, the faster the coagulation of the colloidal solution.