MW
TTh
MW
Student Name _______________________________ _______________
Post-Lab Questions:
am
Determination of Avogadro’s Number
mid
pm
Lab Section (circle one)
Show your complete work and use significant figures properly. Consider drops counted as exact numbers.
1. Calculate the area of the water surface (in cm2) from the measured diameter of the watch glass.
2. Based on the AVERAGE number of drops required to form the monolayer and the AVERAGE number of drops in
1 mL of hexane, calculate the volume of the stearic acid solution delivered (in mL) to the water surface.
3. What was the total mass (in grams) of stearic acid added to the water surface? Use the volume of the solution
from above (in #2) in combination with the concentration of the stearic acid solution (reported on the bottle in
g/mL).
4. Calculate the volume of pure stearic acid in this monolayer (in cm3), after the hexane solvent evaporated.
Note: The density of bulk stearic acid is approximately 0.847 g/cm3
5. Assume the monolayer takes on a thin, cylindrical shape above the water surface. Based on the volume of
stearic acid in #4 above, calculate the experimental height of the monolayer, H (in cm).
Note: Volume of a Cylinder = πr2H = (Area of Circle)·(Height)
6. Calculate W in cm. Using the molecular proportions from the computer model of
stearic acid, relate the experimental height (H) of the stearic acid monolayer
(calculated in #5 above) to the width (W). For the purposes of this calculation, we will
assume the stearic acid molecule can be treated as a rectangular block.
7. Determine the area, W2 (in cm2), for an individual stearic acid molecule aligned on the
surface of water.
8. Based on the total area of the water surface, and the area per stearic acid molecule (W2), how many stearic acid
molecules are in the monolayer?
9. Using the mass of the stearic acid from question #3, how many MOLES of stearic acid molecules are on the
monolayer surface?
10. Using the results from question #8 & #9 above, estimate Avogadro’s number (in molecules/mol).
11. Assume the area you calculated for a single stearic acid molecule was underestimated. How would this error
affect your determination of Avogadro’s number? Would it be overestimated, underestimated or remain
unaffected? Explain your reasoning.