Materials:
Apparatus
Large tray (at least 40 cm wide)
Graph paper with 1 cm squares
Overhead transparencies
Overhead transparency pens
Medicine dropper
Graduated cylinder (10 mL)
Beaker (600 mL)
Safety goggles
Lab apron
Reagent
Oleic acid solution (0.50% dissolved in methanol)
Lycopodium powder (or zinc stearate powder)
Strips of newspaper approx. 5 cm wide and longer than the tray
Procedure:
1 Put on your lab apron and safety goggles.
2 Obtain a medicine dropper and use it and a graduated cylinder to determine how many drops will make up 1 cm3 of water. Repeat two more times. Record in your copy of Table 1 in your notebook.
3 Bring a tray to your lab table. Fill a 600 mL beaker with water and put the water into the tray. If you need more water, get the water in the beaker and pour it into the tray.
4 Allow the water to settle down so that you have a calm surface of water before going on to the next step. Pass a strip of newspaper over the surface to remove unwanted dust.
5 Sprinkle a very fine layer of lycopodium powder on the entire surface of the water. If the powder is moving around, you have not let the surface of the water become calm enough. Wait until all is calm.
6 Put one drop of the oleic acid solution on the water in the middle of the tray. The powder should move away, leaving an area that is clearly defined. Place an overhead transparency above the thin layer of oleic acid and accurately trace the shape of the oleic acid layer.
7 Using graph paper with 1 cm squares, determine the area of the thin layer of oleic acid. Large areas can be squared off and measured, while the edges of the layer can be estimated. The area of one square on the graph paper is equal to 1 cm2.
8 If you have enough time, repeat Steps 3-7. Be sure that you have removed all of the powder from the tray before starting over.
9 When cleaning up, REMEMBER NOT TO CARRY A TRAY FILLED WITH WATER ACROSS THE ROOM. Instead, remove the water at your lab table.
10 Before you lead the laboratory, wash your hands thoroughly with soap and water; use a fingernail brush to clean under your fingernails.
Reagent Disposal:
The oleic acid dissolved in CH3OH should be poured into the designated waste container. The water with oleic acid film can likewise be rinsed down the sink with plenty of water. Place any solids in the designated waste container.
Post Lab Discussion:
In the calculations for this experiment, you will be asked to progress from the number of drops in 1 cm3 to the size of a molecule. A few hints will make the calculations more understandable:
1 In calculating the volume of one drop, remember that the units that you have recorded are in drops/cm3. In order to find the volume of one drop, the units will need to be cm3/drop.
2 Since the oleic acid that is used in this experiment is very dilute, the actual volume of pure oleic acid in one drop will be very small. Remember that the solution is 0.50% oleic acid.
3 Once you know the volume of pure oleic acid, you can figure out the thickness as you did in the previous experiment when you found the thickness of aluminum foil.
Data and Observations:
Trial 1
Trial 2
Trial 3
Number of drops in 1 cm3
Area of oleic acid layer in cm2
Questions and Calculations:
1] Calculate the average number of drops from your medicine dropper that add up to a volume of 1 cm3.
2] Calculate the volume of one drop of liquid. Express your answer in exponential notation.
3] Calculate the volume of pure oleic acid in one drop of 0.50% oleic acid solution. Express in exponential notation.
4] Calculate the thickness of one molecule of oleic acid by dividing the volume calculated in item 3 by the area of the oleic acid layer that you determined in Step 7 of the Procedure. Express your answer in exponential notation.
5] Calculate the volume of one molecule, assuming that the length and width are each one tenth of the thickness. Express in exponential notation.
6] Calculate the mass of one molecule of oleic acid by multiplying the volume by the density.
7] How many significant figures do you have in all of your calculations?
8] Compare your answers with others in your class. How close are the values? Remember that you can only depend on one significant figure in all of the calculations.
Follow-up Questions:
1 Do you think an atom will be bigger or smaller than the value that you calculated for the size of an oleic acid molecule? Explain.
2 Approximately how many molecules of oleic acid were in the drop of oleic acid solution? Remember that you know the volume of one molecule and you know the total volume of pure oleic acid that you used.
3 Using the values that you obtained for thickness, what area (in cm2) would be covered in 10 cm3 of pure oleic acid spread over a large body of water? What is the area in km2?
Questions? Comments??
Yvonne Chan