Explain the method to determine the size of a molecule of oleic acid.

(N/A) Oleic acid is a soapy liquid, and the size of its molecule is of the order of $10^{-9} \,m$.
In this method, the thickness of a molecular layer is determined, which gives the dimension of the molecule.
First, $1 \,cm^{3}$ of oleic acid is dissolved in alcohol to make a solution of $20 \,cm^{3}$. Then, $1 \,cm^{3}$ of this solution is further diluted with alcohol to make $20 \,cm^{3}$.
Thus, the concentration of oleic acid in the final solution is $\left(\frac{1}{20 \times 20}\right) \,cm^{3}$ of oleic acid per $cm^{3}$ of solution.
Water is taken in a large shallow trough, and lycopodium powder is sprinkled on its surface.
When a drop of the oleic acid solution is placed on the water surface, it spreads into a thin, circular film of thickness equal to one molecular diameter.
Let $n$ be the number of drops placed on the water, and $V$ be the volume of each drop in $cm^{3}$.
The total volume of oleic acid in the film is $V_{total} = n \times V \times \left(\frac{1}{20 \times 20}\right) \,cm^{3}$.
If $A$ is the area of the circular film formed on the water, the thickness $t$ of the film is given by $t = \frac{V_{total}}{A} = \frac{n V}{400 A} \,cm$.
Since this film is one molecule thick, $t$ represents the size (diameter) of the oleic acid molecule, which is found to be of the order of $10^{-9} \,m$.