Measuring the Size of a Molecule


Using tools readily available in a high school chemistry lab you will measure the length and the mass of a molecule of the fatty acid from olive oil, known as oleic acid (or cis-9-octadecenoic acid). Oleic acid’s formula is C18H34O2 and its structure is shown below. Our goals:

  1. To determine the length (long dimension) and width (short dimension) of oleic acid molecules
  2. To determine the number of individual molecules in a single drop of pure oleic acid
  3. To determine the mass of a single molecule
oleic.acid (5K)

Molecules of vegetable oil belong to a class of biological molecules called lipids. These are commonly called ‘fat molecules’. Their structure is such that they are poorly soluble in water. This is a good thing because they are the building blocks of cell membranes and without this property we would all be little more than puddles of chemically interesting goo. You will take advantage of their insolubility in water in this lab.

Molecules that dissolve in water are called hydrophilic. The word comes from Greek words meaning ‘water-loving’. Molecules that do not dissolve in water are called hydrophobic. Hydrophobic comes from Greek words meaning ‘water-fearing’. Fatty acid molecules such as oleic acid are interesting because they have parts which are hydrophilic and parts which are hydrophobic.

In the picture of oleic acid at right note that the hydrophilic end is at the bottom. This was done on purpose because when oleic acid molecules encounter water they stand up on it with this end down. Oleic acid floats on water because its density is 0.890 g/cm3 while that of water is very near to 1.0 g/cm3.

The hydrophobic end of the molecule is really the entire rest of the molecule. Because it is so much larger than the hydrophilic end, the entire molecule is not soluble in water. When placed on water oleic acid molecules will stand up and support one another on end because of attractive forces between the hydrophobic parts of the molecules.

If oleic acid is dropped onto water it spreads out to a thickness of only one molecule. In your caculations it will be useful to assume that oleic acid molecules are 4 times as tall as they are wide and that the base of the molecule is a square.

Mathematical Toolkit
V = A × H for shapes with straight sides
D = m/V
dimensional analysis
  1. Carefully fill a dropper with methanol. This solvent has a high vapor pressure and will build up pressure inside the dropper, pushing the it out. When you fill it do so by repeatedly drawing methanol in and out of it to reduce the build-up of pressure. Store the pipet in a beaker to contain drips.
  2. Count the number of equal-sized drops it takes to equal a volume of just over 1 mL in a 10-mL graduated cylinder. Be sure to read the volume exactly, to the nearest 0.01 mL and do not try to make the volume you measure equal 1.00 mL exactly.
  3. Calculate the volume of a single drop of methanol and write it down.
  4. Fill the tray 2 cm deep with H2O.
  5. Form a very thin layer of lycopodium powder on the surface of the H2O. Try to do this in a way that is repeatable.
  6. Fill the same dropper with oleic acid solution. Use the same procedure as with the pure methanol.
  7. Place a single drop on the lycopodium-covered water. Oleic acid dissolves in methanol and methanol dissolves in water. When you drop the solution onto the water the methanol dissolves in the water and all that is left on the surface is oleic acid.
  8. Observe what happens and write down your observations.
  9. Experience has shown that the spot tends to grow after a while. This may be due to piled-up oleic acid molecules spreading out. Consider: what effect will it have on your determination of the length of a molecule if you measure the area when it has piled-up molecules? What effect would it have on your determination if you measure the area after it has spread out beyond the limits of the area actually covered by the monolayer of oleic acid?
  10. Use the transparency film and marker to trace the perimeter of the spot of oleic acid.
  11. Lay the transparency over the graph paper and determine the area in square centimeters (cm2).
  12. Repeat the procedure at least three times in order to be able to provide an average, range and precision. More repetitions may be necessary as determined by the quality of your data. Calculate the length of a molecule using your results from each trial while you are still in the lab. Using that information decide whether you would like to do another trial. Perhaps you might change something about your technique: if so, be sure to record this in your notebook.
Do the following calculations in your lab notebook. Everyone is responsible for doing the calculations.
  1. Use the concentration of oleic acid (3.6 × 10-3 g/mL) and the density of oleic acid (0.890 g/cm3) to determine the mass and volume of pure oleic acid in one drop of the methanol solution. You determined the size of the drop in step two above.
  2. Using the volume formula relating volume, area, and height, find the height of the spot of oleic acid. Use the volume of pure oleic acid determined in the previous step. Since the oil spreads out to an area only one molecule thick, this is the length of the molecule. Report your answer in cm and nm.
  3. Calculate the width of the molecule in cm and nm. Assume that the molecule is 1/4 as wide as it is tall.
  4. Assuming that the base of the molecule is a square, find the area of the base of the molecule in cm2. Use that result to calculate the number of molecules in the spot of oleic acid.
  5. Calculate the mass of one molecule using the mass of oleic acid in one drop (the mass of oleic acid in the spot) and the number of molecules.
  6. Find the precision your measurements of the length and mass of oleic acid molecules. Express the precision as a percent error by dividing the ± amount by the size of the measurement. The width will have the same precision and accuracy as the length measurement.


Do a full formal lab write-up. Include in your report all of the caculations asked for in the Discussion section. Do this in a dedicated section titled Sample Calculations. Show all steps and include units at each stage of the calculation. Include two data tables. The first should show your precision in measuring the volume of a drop from your dropper. This will allow you to discuss this as a source of error. The second must show the mass of oleic acid in the spot (g), length of the molecule (nm), number of molecules in the spot, and the mass of one molecule (g). Give averages for length and mass and calculate experimental precision based on the range in these results. Also, compare your results with the data below. Do a comparison which gauges your accuracy (see method below).

Oleic Acid C18H34O2 282.47 g/mol
Length1.97 nm
Mass4.69 × 10-22 g
These data are provided in order for you to decide whether your experimental results are accurate. You can calculate a percent error for accuracy by using the following formula:
            experimental value – literature value
% error = ———————————————————————————————————————— × 100%
                      literature value
If your result is too high then you will have a positive % error.
Source for these data and the density of oleic acid:, search on ‘oleic acid’.
Idea for this lab is courtesy of Dr. Richard Lewis, Scarborough High School.
There is a sheet of exercises to help students prepare to do this lab here.
Last updated: Oct 18, 2011 Home