An Experiment to Determine the Molecular Size

• Requirements: Lycopodium powder, clean water, olive oil,a tray, microscope, wire.
• Procedure: Alittle of lycopodium powder is sprinkled over the surface of some clean water contained in a tray. A small drop of olive oilof radius r is then putted on the surface of the water with help of a wire. The olive oil is then spread to form a circular film of one molecule thick. The lycopodium powder enables the edges of the oil to be clearly seen.
• Calculation:

let r = radius of drop,

a = radius of film and t = tickness of film

The volume of film = volume of drop

I.e. πr³t = 4 πr³ /3              therefore, t= 4πr³3r²

Generally, the thickness of a molecule is given by; thickness= VolumeArea

Assumptions:

5. When the drop spread to a maximum then the film can be on molecule thick.
5. The space between the molecules is ignored.

Example: Calculate the thickness of a molecule of volume 2.0*10^-3m³, whose diameter is 4.0*10^-2m.

Solution

t= VA= Vπr             But Vπ=2.0*10^-3m³ = 22/7

r= d2= 4.0*10-2m2=2.0*10-3m 

t= 2.0*10-3m322/7(0.02m)²=2.0*10-3m33.14*4.0*10-4 

t = 1.67m