Question

The coagulation of 200 mL of a positive colloid took place when 0.73g HCl was added to it. The flocculation value of HCl for the colloid is:
A. 150
B. 200
C. 100
D. 36.5

Answer 1

The minimum concentration of electrolyte required to cause coagulation of sol is called as Flocculation value which is calculated in terms of millimoles per liter. The positive charge will show maximum flocculation with a negative charged ions.

Complete step by step answer: The volume of positive colloids is 200 mL.
Mass of HCl is 0.73 g.
The flocculation value is defined as the minimum concentration of electrolyte used to cause coagulation of sol. The flocculation value is calculated in terms of millimoles per litre.
The formula to calculate the number of moles is given below.
$n = \dfrac{m}{M}$
n is number of moles.
m is given mass.
M is molecular weight

The molecular weight of hydrochloric acid is 36.46 g/mol.
To calculate the moles of hydrochloric acid, substitute the values in the above equation.
$\Rightarrow n = \dfrac{{0.73g}}{{36.46g/mol}}$
$\Rightarrow n = 0.020mol$
The value in mole is converted into millimole.
1 mole is equal to 1000 millimole.
0.020 mol is equal to $\Rightarrow 0.020 \times 1000 = 20mmol$
For 200 mL of the positive colloid solution 20 mmol of HCl is required to cause flocculation.
For 1000 mL (1 L) of the positive colloid solution, the HCl required is
$\Rightarrow \dfrac{{20}}{{200}} \times 1000 = 100mmol$
Thus, the flocculation value of HCl for the colloid is 100mmol/L.
Therefore, the correct option is C.

Note:
According to Hardy-Schulze rule “Greater is the valency of ions, greater will be the power to bring coagulation”. The flocculation value is inversely proportional to coagulation power.