Question

The average molecular mass of colloids can be determined by:
(a)- Tyndall effect
(b)- Brownian movement
(c)- Osmotic pressure
(d)- Flocculation

Answer 1

The average molecular mass of the colloids can be calculated by the formula $M=\dfrac{wRT}{\pi V}$ where, w is the colloid particles, R is the solution constant, T is the temperature of colloidal solution, V is the volume, and $\pi$ is the pressure.

Complete answer:
We can calculate the molecular mass of the colloid by osmotic pressure.
The osmotic pressure ($\pi$) of the colloidal solution is directly proportional to the concentration of the solution (C) and its temperature (T). Mathematically,
$\pi \propto C$
$\pi \propto T$
$\pi =\text{ R x T x C}$
Where R is a constant (solution constant) and its value is found to be the same as that of the Gas constant.
$\pi =CRT$
But, $C=\dfrac{n}{V}$
Where n is the number of moles of sol, and Vis the volume of the solution.
$\pi =\dfrac{n}{V}RT\ \text{ or }\pi V=nRT$
If w grams of sol are dissolved in V liter of the solution and M is the average molecular mass of the sol then,
$n=\dfrac{w}{M}$
Substituting this value in the equation, we get
$\pi V=\dfrac{w}{M}RT$
$M=\dfrac{wRT}{\pi V}$
So, with osmotic pressure, we can calculate the average molecular mass.
This method is not usually preferred over the determination of molecular masses because of many experimental difficulties involved. However, for the determination of molecular masses of proteins, polymers, and other macromolecules like the colloidal solution, it is considered as one of the most suitable methods.

Hence the correct answer is an option (c)- Osmotic pressure.

Note:
The particles of a colloidal solution are simple molecules, but a physical aggregation of molecules. Their osmotic pressure will be low because their numbers of particles are very small.