Summer Research Program for Science Teachers
Nightingale-Bamford School, Manhattan
How Big is a Molecule?
[9-12 Content Standard A- Identify questions/concepts that guide inquiry]
The purpose of
this activity is to estimate the size of a molecule of oleic
Background: An oleic
acid molecule is a straight-chained organic
As you probably know,
a molecules 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]
oleic acid solution (5% by volume)
10-ml graduated cylinder
PC or Macintosh with spreadsheet software (optional)
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
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
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.
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.
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:
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?
your results to estimate the number of oleic acid molecules
present in your monolayer.
we had used 10% oleic acid solution instead of 5% solution, would
the results of the experiment been different? Explain why
or why not.
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.
were the flaws in the experimental design that limited the
accuracy of the estimated oleic acid molecule length?
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