Measuring the Size of a Molecule

Objective

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)
Background

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.


Materials

Mathematical Toolkit
V = A × H for shapes with straight sides
D = m/V
dimensional analysis


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Procedure
  1. Carefully fill a dropper with methanol and count the number of equal-sized drops it takes to equal a volume of 1 mL (1 cm3). (Note: Methanol is poisonous and should be washed off of your hands immediately if spilled. You are using just methanol because it is troublesome to mix up the oleic acid solution and it would be wasteful to use it now.)
  2. Calculate the volume of a single drop of methanol and write it down.
  3. Fill the tray partway with H2O.
  4. Form a thin layer of lycopodium powder on the surface of the H2O.
  5. Use the same dropper now partially filled with oleic acid solution to 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.
  6. Observe what happens and write down your observations.
  7. Use the transparency film and marker to trace the perimeter of the spot of oleic acid.
  8. Lay the transparency over the graph paper and determine the area in square centimeters (cm2).
  9. Repeat the procedure three times in order to be able to provide an average, range and precision.

Discussion
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, width and mass of oleic acid molecules. Express the precision as a percent error by dividing the ± amount by the size of the measurement.

Grading

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. As usual, the report should be typed: 12 pt Times New Roman, 1 inch margins, 1.5 line spacing. Also include a data table showing volume of oleic acid in the spot (cm3), mass of oleic acid in the spot (g), length of the molecule (nm), width of the molecule (nm), number of molecules in the spot, and the mass of one molecule (g). Give averages for length, width, 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
Width0.5 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: www.chemexper.com, search on ‘oleic acid’.
Idea for and design of 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 12, 2009 Home