Question

# In 1774 Benjamin Franklin described some experiments he had performed on a pond at Clapham Common...

In 1774 Benjamin Franklin described some experiments he had performed on a pond at Clapham Common in London. He observed that a teaspoon (volume 4.5 cm3) of olive oil [Oleic Acid CH3(CH2)7CH=CH(CH2)7COOH ; density 0.92 g cm-3] when spread of a water surface of half an acre (2 x 103 m2) would calm that entire surface.
Calculate the thickness (in nm) of the surface film and the surface area (in nm2) occupied by an oleic acid molecule on water. Account briefly for your answers.

Part a

Volume of oleic acid V = 4.5 cm3 x 10^-6 m3/cm3

V = 4.5 x 10^-6 m3

Area of surface A = 2 x 10^3 m2

Thickness of surface film = V/A

= 4.5 x 10^-6 m3 / 2 x 10^3 m2

= 2.25 x 10^-9 m x 10^9 nm/m

= 2.25 nm

Part b

Avogadro number = 6.022 x 10^23 molecules / mol

6.022*10^23 molecules cover = 2000 m2

1 molecule will cover = 2000 m2 / 6.022*10^23

= 3.321 x 10^-21 m2

surface area (in nm2) occupied by an oleic acid molecule

= 3.321 x 10^-21 m2 x (10^9 nm/m)^2

= 0.003321 nm2

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