Background
In this experiment you will estimate the number of stearic acid molecules in a mole of stearic acid
based on an assumption about a thin film of stearic acid on a water surface. A monolayer of stearic
acid can be created in which we can estimate the number of molecules. To do this we make assump-
tions of the size and shape of the stearic acid molecule.
In the International System (SI) of units, the fundamental measure of quantity is the mole. A mole
of carbon is 12.0000 grams of carbon-12, and Avogadro’s number, NA, is the number of carbon
atoms in this mass. A mole of any substance contains Avogadro’s number of particles.
In this experiment you will estimate Avogadro’s number by calculating the amount of stearic acid
necessary to form a single layer on the surface of water. This single layer is called a monolayer. We
will be making some assumptions about how stearic acid molecules arrange themselves to form the
monolayer. From these assumptions we can determine the thickness of the layer, and from the
thickness we can estimate the number of the stearic acid molecules. If we know the volume occu-
pied by a mole of stearic acid molecules we can use the volume of one stearic acid molecule to esti-
mate Avogadro’s number. We simply divide the volume of a mole of stearic acid by the volume of
one stearic acid molecule.
Note – the number calculated will not be accurate, but should be within a power of 10 of the
accepted value of 6.02214 x 1023 particles/mol.
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