Summer Research Program for Science Teachers

Todd Flomberg

Nightingale-Bamford School, Manhattan



How Big is a Molecule?

[9-12 Content Standard A- Identify questions/concepts that guide inquiry]


Objective:       The purpose of this activity is to estimate the size of a molecule of oleic acid.

Background:   An oleic acid molecule is a straight-chained organic molecule.  One end of the molecule, the polar-head group, is attracted to water (hydrophilic), the remainder of the chain is non-polar points away from the surface and into the air.  It is hydrophobic.  The head-groups that remain in the water form a monolayer.  [9-12 Content Standard B- Structure and properties of matter] A monolayer is a film that is one molecule thick.

As you probably know, a molecule’s volume is too small to measure by ordinary means.  During this activity you will estimate the length of an oleic acid molecule by dividing the volume of one drop by the area of the monolayer.  The length of the oleic acid molecule is the depth of the monolayer.


volume = area * depth


depth = volume/area

[Content Standard Unifying Concepts- Change, constancy, and measurement]


Materials:       chalk dust

                        oleic acid solution (5% by volume)

                        10-ml graduated cylinder




                        PC or Macintosh with spreadsheet software (optional)

[Teaching Standard D- Make accessible science materials]


I.                    Pour water into the tray to a depth of about 1-cm.  Spread a small amount of chalk dust very lightly across the surface of the water.  Be careful not to use too much dust or else the oleic acid will be hemmed in and not spread to form a uniform monolayer.


II.                 Using the eyedropper, gently place one drop of 5% oleic acid solution to the surface of the water.  When the drop touches the water the alcohol in the solution will dissolve in water, but the oleic acid will float and spread out to make a circular monolayer.


III.               Measure the diameter of the circle in three different places and calculate the average diameter of the oleic acid monolayer in units of centimeters.  Use this data to calculate the area of the circle (Area = pi * r2).  Make sure to use the proper number of significant digits.


IV.              Count the number of drops of oleic acid solution it takes to occupy 3-ml (or 3 cm3).  Repeat this twice more and calculate the average number of drops in 3-ml of solution.  Use this information to calculate the average volume of one drop of oleic acid solution in cubic centimeters.  Make sure to use the proper number of significant digits.


V.                 Before calculating the length of an oleic acid molecule recall that your oleic acid solution is only 5% oleic acid, the rest is water.  Recalculate the volume of an oleic acid molecule, then estimate the length of the oleic acid molecule by dividing the volume by the area of the monolayer.  Convert your answer to both nanometers and Angstroms.


Questions and Analysis:

Use a chemistry text or the Internet to learn more about the diameter of an atom.  How does your answer for the length of an oleic acid molecule compare to established estimates of the atomic radius of a hydrogen atom? [9-12 Content Standard A- Formulate and revise explanations using evidence]

Use your results to estimate the number of oleic acid molecules present in your monolayer.

If we had used 10% oleic acid solution instead of 5% solution, would the results of the experiment been different?  Explain why or why not.

Explain why it was necessary to dilute the oleic acid when trying to form a monolayer.  Think about how the molecules are arranged and what might happen if we used 100% oleic acid instead.

What were the flaws in the experimental design that limited the accuracy of the estimated oleic acid molecule length?


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