Avogadro’s Number
Purpose

To determine the experimental value for Avogadro’s number
Introduction
Avogadro’s number is the number of atoms, molecules, or formula units present in
one mole of a substance. It is the number of 12C atoms present in exactly 12 g of the C-12
isotope. It is also the number of Cu atoms in 63.55 g of copper, the number of water
molecules in 18.02 g of water and the number of formula units in 58.44 g of NaCl. It is a
very large number, 6.02 X 1023, because the particles being counted are extremely small.
In this experiment, the number will be calculated by spreading a film of a fatty
substance only one molecule thick (monolayer) onto a water surface. Fatty acids, named
because of their presence in many natural fats, are used for this purpose because their
molecules contain a long hydrocarbon chain (nonpolar) and a carboxylic acid end (polar).
Because of the hydrocarbon chain, these molecules are generally insoluble in water and will
float on the surface, with the polar end attracted to water and the nonpolar end repelled by
water.
Stearic acid, C17H35COOH dissolved in hexane, a volatile hydrocarbon, will be used
for this experiment. When a drop of this solution is placed on water, the stearic acid spreads
out on the surface, leaving a monolayer as the hexane evaporates. Additional drops will
cause the monolayer to spread out until the entire water surface is covered. One extra drop
will cause a second layer to float on top of the monolayer forming a lens-shaped droplet that
is visible. When this lens is observed, you have gone too far.
Procedure
Part I Calibration of Pasteur Pipet
1. Place about 5 mL of hexane in a clean, dry test tube. Calibrate the pipet by adding
hexane one drop at a time into a clean, dry 10 mL graduated cylinder. Hold the pipet in
a vertical position so that the drops are uniform. Record the number of drops needed to
reach the 2.0 mL mark.
2. Repeat the calibration procedure two more times. The number of drops per mL should
agree to within 2-3 drops. Dispose of the hexane in proper container, not in the sink.
Part II Determination of Avogadro’s Number
1. Measure the inside diameter of a petri dish to the nearest 0.1 cm.
2. Thoroughly clean the petri dish with glassware detergent and rinse in cold water for at
least 2 minutes. The dish must be absolutely clean. Do not touch the inside surface
after cleaning.
3. Fill the petri dish completely with distilled water.
4. Obtain 1.0 mL of stearic acid solution in a clean, dry test tube. Record the
concentration of the solution.
5. Rinse the dropper pipet by drawing some of this solution into it and discarding the
contents.
6.
7.
8.
Fill the pipet with stearic acid solution.
Add the solution, dropwise, onto the water surface in the dish. Allow about 10 seconds
between drops for the stearic acid to spread over the water surface and for the solvent to
evaporate. A second layer is formed when a drop no longer spreads but instead persists
for about 30 seconds in the shape of a lens on the surface. Record the number of drops
required to form only the monolayer.
Repeat the procedure two more times. Dispose of the unused stearic acid solution in
the proper container.
Calculations
Part I Calibration of the Pasteur Pipet
1. Calculate the average number of drops per mL
Part II Determination of Avogadro’s Number
1. Calculate the surface area of the monolayer in the petri dish. Express the value in
square nanometers.
2. Calculate the number of stearic acid molecules in the monolayer, assuming the area
each stearic acid molecule covers is 0.21 nm2 and there are no spaces between the
molecules.
3. Calculate the volume of stearic acid solution required to form the monolayer, based on
the average number of drops used.
4. Calculate the mass of stearic acid, using the concentration of the solution given in
g/mL.
5. Calculate the number of moles of stearic acid. The molar mass = 284.5 g/mole.
6. Calculate the number of stearic acid molecules per mole. This is Avogadro’s number.
7. Calculate percent error.
Data
Part I
Trial 1
Number of drops in 2 mL
Part II
Concentration of stearic acid
Diameter of petri dish
Drops required for monolayer
Trial 2
Trial 3
Results
Part I
Average number of drops per mL
Part II
Surface area of petri dish
Number of molecules in monolayer
Volume of stearic acid solution in monolayer
Mass of stearic acid in monolayer
Moles of stearic acid in monolayer
Avogadro’s number
Percent error
Questions
1. List three sources of error and tell how each one affected the calculated value. Explain.
2.
Using the chemical characteristics of the substances used in this experiment, explain
how the monolayer was formed.