The Calculation

The estimate of the size of a molecule and Avogadro's number goes something like this.

1. V = volume of oil = 1 tsp = 1 1/3dram = 4.93 cm^3.
   
2. A = area of oil = 1/2 acre = 1/2 x 4.047 x 10^7 cm^2 = 2.024 x 10^7 cm^2.
   
3. D = diameter of a molecule = volume/area = 4.93 cm^3/2.024 x 10^7 cm^2 = 2.44 x 10^-7 cm = 24.4 Angstroms.
   
4. Assume close packed, spherical molecules in a monomolecular film or layer.
   
5. Number of molecules in the monomolecular layer = A/Pi (D^2)/4 = 4.3 x 10^20 molecules.
   
6. The weight of a monolayer of oil = density x volume of oil.
   
7. mass of oil = 0.964 g/cm^3 x 4.93 cm^3 = 4.75 g.
   
8. Now a real assumption using a molecular weight in grams per mole.
   
9. Moles of oil in the weight of oil = 4.75 g/226g/mol = 0.0238 moles.
   
10. Number of molecules per mole = Avogadro's number = 4.3 x 10^20 molecules/0.024 moles = 1.8 x 10^22 molecules per mole.
    

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