Numero De Avogadro

Dr. Michael Drake, Dr. Claudia Brackett
California State University Stanislaus
Dept. of Chemistry
Determination of Avogadro’s Number
Objective
The object of this experiment is to experimentally determine the value of Avogadro’s
number, to recognize possible experimental errors and to evaluate the effect of these
errors on the experimental results.

Introduction
Amedeo Avogadro, or moreproperly, Lorenzo Romano Amedeo Carlo Avogadro
di Quaregna e di Cerreto, Count of Quaregna and Cerreto (August 9, 1776 – July 9, 1856)
was a member of the Italian nobility, a professor of Physics at the University of Turin,
and an eminent scientist. Avogadro is most noted for his contributions to the theory of
molarity and molecular weight which were formulated partially based on Avogadro'sLaw. Avogadro's Law, which was published in 1811, states that the relationship between
the masses of the same volume of different gases (at the same temperature and pressure)
corresponds to the relationship between their respective molecular weights. Hence, the
relative molecular mass of a gas can be calculated from the mass of a sample of known
volume.
Through the work of a number of otherscientists, especially the work of Stanislao
Cannizzaro, Avogadro’s Law led to the establishment of the mole concept. A mole is
defined as the amount of substance that contains as many elementary particles as there
are atoms in exactly 12.00 grams of the carbon-12 (12C) isotope. In honor of Avogadro,
the number of elementary particles (atoms, molecules, or ions) in one mole of a
substance,6.022142 x 1023 particles/mol, is known as Avogadro's number.
There are several methods for the experimental determination of Avogadro’s
number. In this experiment you will estimate this number by an indirect count of the
number of molecules present in a monolayer film of fatty acid molecules on water.
Stearic acid, which has a molecular formula of C17H35CO2H, will be used to form the
monolayer.Stearic acid (Figure 1) has a long hydrophobic (water hating) carbon chain
(the C17H35- portion of the molecule) and a hydrophilic (water loving) carboxyl group
(the -CO2H portion of the molecule).

Figure 1: stearic acid, C17H35CO2H

Solid stearic acid, which is a component of vegetable shortening, is insoluble in
water because of its long hydrophobic tail. It is, however, soluble inorganic solvents
such as hexane. To form the desired monolayer, a hexane solution of stearic acid is made
and added drop wise to the surface of the water contained in a petri dish. The hydrophilic
carboxylic acid group, -CO2H, is drawn to the water surface while the hydrophobic
hydrocarbon group, C17H35-, is repelled by the water and sticks out from the water
surface (Figure 2). Hexane has a highvapor pressure and evaporates quickly leaving
behind a solid fatty acid film one molecule thick on the surface of the water. If the right
number of drops of the fatty acid-hexane solution is added, the entire water surface will
be covered with a unimolecular layer of stearic acid.



Figure 2: A diagram of stearic acid molecules lined up in a monolayer on a water
surface.

A singlestearic acid molecule is known to occupy an area of 0.21 nm2. Thus, if
the surface area of the Petri dish is known, the number of stearic acid molecules in the
monolayer can be calculated. Then, from the volume of hexane-stearic acid solution
added to form the monolayer and the known concentration of the solution it is possible to
determine the number of moles of stearic acid added. Using thesetwo experimentally
measured numbers, moles of stearic acid and molecules of stearic acid, it is possible to
calculate the number of molecules of stearic acid in a mole.

Procedure

Note: All waste should be discarded in the waste container provided.

I. Calibration of a Pasteur Pipet
1. Obtain a clean and dry 10 mL graduated cylinder. Obtain approximately 5 mL of
hexane in a separate...