Approximating Avogadro’s Number
Problem:
Approximate the size of a stearic acid molecule using several simple
measurements and the number of stearic acid molecules in a mole of stearic acid.
Introduction:
Stearic acid is a molecule with a long, hydrophobic hydrocarbon chain which
contains a hydrophilic carboxylic acid (COOH) functional group on one end. When
stearic acid solution is placed dropwise onto water, the stearic acid molecules will
arrange so that the carboxylic functional group clings to the water and the hydrocarbon
chain is raised straight above the carboxylic group. We will assume that the stearic acid
layer will be a single molecule in depth and that stearic acid molecules are cubic.
Stearic Acid: C18H36O2
A = πr2
V = πr2 h
V = Ah
A=h2
Materials:
Large plastic tray
meter stick
lycopodium(baby) powder
Stearic acid in hexane (0.12-0.15 g/L) – exact value will be provided in class
Procedure:
1. Obtain a large plastic tray. Thoroughly clean the tray with soap and water and then
rinse well.
2. Fill the tray with water to a depth of at least 1cm in the center of the tray. Using a
shaker bottle, dust the water evenly with just enough lycopodium powder to make a
visible layer.
3. Obtain a dropper and a 3.5 mL sample of stearic acid.
4. Being careful not to disturb the tray, add one drop of stearic acid to the center of the
tray. Very quickly measure the diameter of the stearic acid monolayer by using a
meter stick. Make four measurements with the meter stick held in a different
orientation for each measurement.
5. Discard the contents of the tray and clean and rinse the tray. Repeat the experiment
two more times.
6. Clean the tray.
7. Find the number of drops of stearic acid needed to dispense a volume of 1.00mL.
Repeat this procedure three times.
Data:
Create a data table which will allow you to report all measurements made to complete
the experiment. The data table should include all units, measurement uncertainty and the
average value for the diameter of the stearic acid monolayer and the average number of drops
of stearic acid in 1.00mL.
Analysis and Conclusions:
1. Calculate the molar mass of stearic acid.
2. Determine the average volume of the stearic acid monolayer by using information
determined in step 7 of the procedure.
3. Calculate the average area of the stearic acid monolayer by using the data collected in
steps 4-5.
4. Calculate the height of the stearic acid film. Remember that 1mL = 1cm3
5. Using the assumption that stearic acid is cubic, calculate the area occupied by one
molecule of stearic acid.
6. Calculate the mass of stearic acid that would be present in an average droplet of
stearic acid solution.
7. Calculate how many molecules were present in the stearic acid monolayer (one drop).
8. Convert the mass of stearic acid in one droplet into moles.
9. Calculate your approximation of Avogadro’s number to determine how many
molecules of stearic acid there are per mole. Use results from #7 and #8.
10. Determine the percent error of your experimental approximation of Avogadro’s
number.
11. Identify sources of error and discuss their significance, i.e. will they cause your
experimental value for Avogadro’s number to be higher or lower than the accepted
6.022 x 1023 and why?
Name_________________________
Individual Pre-Lab Questions – Answer all questions in your own words!
1. Google “MSDS Stearic Acid” and “MSDS Cyclohexane,” what safety concerns are
associated with these chemicals and what guidelines should you follow when completing
this lab?
2. Calculate the molar mass of stearic acid C18H36O2.
3. How will you use your experimental data to calculate the average volume of the
stearic acid?
4. Describe how you will calculate the area of the stearic acid monolayer or provide a
formula.
5. How will determine the height of the stearic acid molecules or provide a formula.
6. How will you be able to approximate Avogadro’s number using your data?