Measurement of Length Size of Molecule

Description:

To measure a very small size like that of a molecule (10^-9 m), we have to adopt special methods.

We cannot use a meter scale as it has a minimum count of only 1meter, we cannot use Vernier calliper with 10^-4 m minimum count, and screw gauge with minimum count 10^-5 m is also not useful. Even a microscope has certain limitations.

To observe an object, we project light on it and the object acts as a secondary source and reflect the light. Light has a wavelength of 10^-8 m, any object smaller than the wavelength of light will not be visible as the wave will move across the object without any interaction.

So to estimate the size of molecule we perform the following calculations −

Volume of any substance is given by −

V = L × B × W

Where,

L = Length

B = Breadth/thickness

W = Width

∴ B = V/L × W

As we know,

Area = L × W

∴ B = V/Area

Now, for the measurement of size of molecule, we will consider the molecule of Oleic acid as it has large molecular size.

The idea is to first form mono-molecular layer of oleic acid on water surface.

We dilute the oleic acid by alcohol for this we dissolve 1 ml of oleic acid in alcohol to make a solution of 20 ml.

Then we take 1 ml of this solution and dilute it again to 20 ml, using alcohol. So, the concentration of the solution is −

1/20 × 20 oleic acid consentration per ml. of solution

Next we sprinkle some lycopodium powder on the surface of water in a large trough and we put drops of the diluted solution with the help of burette in the water.

The oleic acid drop spreads into a thin layer of molecular thickness on water surface.

Let `V` be the volume of each drop and a total of `n` drops are poured on the water surface, then the total volume of the solution is −

nV

As soon as the solution comes in contact of air, the alcohol molecules evaporate and we obtain a layer purely formed by the molecules of oleic acid.

The volume of acid layer is given by −

Volume = nV × 1/400

Now to get the area of the layer, a graph paper is rolled over the surface of the layer which imprints the shape of the acid layer on to the paper. Using this imprint of acid layer its area is gained.

Hence, thickness of oleic acid molecule is given by −

Thickness = Volume/Area

Thickness = nV/20 × 20 × A