Estimation Of Very Small Distances: Size Of A Molecule » Maths And Physics With Pandey Sir

A simple method for estimating the molecular size of oleic acid. Oleic acid is a soapy liquid with large molecular size of the order of 10⁻⁹ m.

We dissolve 1 cm³ of oleic acid in alcohol to make a solution of 20 cm³, Then we take 1 cm³ of this solution and dilute it to 20 cm³, using alcohol. So the concentration of the solution is 1/20×20 cm³ of oleic acid/cm³ of solution.

Next we lightly sprinkle Some lycopodium powder on the surface of water in a large trough and we put 1 drop of this solution in the water. The oleic acid drop spreads into a thin large and roughly circular film of molecular thickness on water surface. Now we can measure it diameter to find area. Let we have dropped n drops in the water. Initially, we determine the approximate volume of each drop ( V cm³).

Volume of n drops of solution = nV cm³

Amount of oleic acid in this solution = nV [1/20×20] cm³

This solution of oleic acid spreads very fast on the surface of water and forms a very thin layer of thickness t. If this spreads to form film of area A cm², then the thickness of the film

Estimation of very small distances: Size of a molecule

If we assume that the film has mono-molecular thickness, then this becomes the size or diameter of a molecule of oleic acid. The value of this thickness comes out to be of the order of 10⁻⁹ m.