Lab 8 - Determination of the Size of a Molecule
Atoms and molecules are too small to be seen with the naked eye and they are also too small to be observed by
the types of microscopes commonly found in high school science laboratories. In a previous experiment, an
indirect method was used to determine the thickness of a piece of aluminum foil. The area of the foil was
determined by multiplying length by width. The volume of the foil was found by dividing its mass by the
density of aluminum. Finally, thickness was determined by dividing volume by area. In this experiment, the
same technique will be used to find the thickness of a molecule. With the use of some simplifying assumptions,
the approximate mass of a molecule will also be determined.
Oleic acid is an organic compound made of carbon and hydrogen. It is not water-soluble and it is less dense
than water so it will float on the surface of water. Its density is 0.895 g/cm3. Oleic acid will spread out to a
very thin layer when placed on the surface of water. In fact, it is generally thought that oleic acid will spread
out to a thickness of one molecule.
Simplifying Assumptions:
 Oleic acid molecules spread out to a thickness of one molecule.
 The molecule is 10 times tall as it is wide and the base of the molecule is square.
Procedure
1. Obtain a beral pipet and 10-mL graduated cylinder. Determine how many drops of water are in 1 cm3 of
water. Record results. Repeat twice, recording data each time.
2. Obtain a tray. Use the 600-mL beaker to fill the tray about half-full of water.
3. Allow the water to settle so that its surface is calm.
4. Sprinkle a very fine layer of lycopodium powder or zinc stearate over the entire surface of the water.
Sprinkle the powder carefully and avoid inhaling the powder. Allow the water to return to a calm state
before proceeding.
5. Place one drop of oleic acid solution on the powder in the center of the tray. Hold a transparency over
the tray and carefully trace the shape of the oleic acid layer.
6. Using the 1-cm square graph paper, determine the area of the thin layer of oleic acid. The area of each
square on the graph paper is 1 cm2. It will be necessary to estimate the areas of the edges of the layer.
Record the area.
7. Empty and clean the tray. Repeat steps 2-6. Record data.
8. Clean up and wash your hands.
Prelab Questions
Directions: Show your work and express answers with proper units.
1. Find the volume of a single drop of liquid given that 20 drops = 1 cm3.
2. What volume of oleic acid is in 1 cm3 of 0.50% oleic acid solution? (I.e., there is 0.50 cm 3 of pure oleic
acid in each 100 cm3 of solution.)
3. What volume of pure oleic acid is in one drop of 0.50% oleic acid solution? (20 drops = 1 cm3)
4. Describe how to find the area of an irregularly shaped thin layer of oleic acid using transparencies and 1-cm
square graph paper.
5. Calculate the thickness of a material that spreads over an area of 3200 cm2 with a volume of 0.050 cm3.
Analysis
Directions: Perform the calculations twice – once for each of your two trials. Show your work and express
answers with proper units. (Hint: Use what you learned in your pre-lab questions.)
1. Calculate the average number of drops in a volume of 1cm3. Express the value as drops/cm3.
2. Using your answer to #1 above, calculate the volume of 1 drop of liquid. In other words, find the reciprocal
in cm3/drop.
3. Calculate the volume of pure oleic acid in one drop of oleic acid solution. Remember that the oleic acid
solution is 0.5% oleic acid.
4. Calculate the thickness of one molecule of oleic acid. Recall that T = V/A and that you determined the area
of the oleic acid layer in step 6 of the procedure.
5. Assume that the length and width of the molecule are each one-tenth of its thickness. Calculate the volume
of one molecule of oleic acid.
6. Calculate the mass of one molecule of oleic acid using its volume and its density of 0.895 g/cm3.
7. Compare your results with classmates. Comment on the accuracy of the results and list reasons for any
discrepancies.
8. Will an atom be larger or smaller than a molecule? Explain your answer.
9. Approximately how many molecules of oleic acid were in the drop of oleic acid solution? Hint: Use the
volume of one molecule and the total volume of oleic acid you used. (Calculate for both trials.)
10. Using the average value for thickness, what area in square centimeters would be covered if 10 cm3 of pure
oleic acid spread over a large body of water? Convert this area to square kilometers.