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        It is instructive to determine the size of a molecule, or at least to get some limit on the size. The following procedure is somewhat simple, and as many other techniques, allows you to measure the size of a molecule indirectly, but in a valid manner so that you can be somewhat confiedent of the results or outcome. It is strongly suggested that before you jump into the experiment that you read the instructions over a few times. You ought to have fun doing this one.

        Oleic acid is an organic acid molecule that is soluble in alcohol, but not in water. As you know, oil and water do not mix. A drop of oil goes a long way. Just look at the problems we had in Prince William Sound with the Exxon Valdez oil spill.     If we were to spill a drop of oil in a swimming pool over at Butler Center (in addition to upsetting people who worked there) it would probably spread out over all the pool and have some left over. The oil will spread out and continue to do so, until it gets to be just one molecule thick. Then it can't spread out anymore. That is why cleaning up an environmental oil spill is such a problem. It really covers everything in sight. because we can't spread a drop over a tremendous sized pool or lake, we are going to dilute the oleic acid (i.e., the oil) so we can measure its effect on a measureable sized container of water. It will be like a pizza pie that is one molecule (of oleic acid oil) thick. So if we can determine the thickness of the pizza we know the thickness or size of a molecule! Simple reasoning and planning can take us a long way.

        The theory is simple, but you must very carefully folow directions or you'll probably just get confused. You will be measuring the thickness of a molecule, about 10^-7 centimetres. That's small, it amy even boggle your mind. But what is more mind boggling is that you can do this with a metre stick! Let's go.

        First of all, like we said, the oilly acid tends to spread out. As I said above, one drop would more than cover an entire swimming pool. So we have to dilute it considerably so there is just a teeny tiny bit of oil in the alcohol. Since the oil is soluble in alcohol, we will dilute it in alcohol and then put an oil/alcohol mixture in an eye-dropper and drop it onto the surface of a container of water. The oil will not mix with water. (Hmmm, where have I heard that before?) To be able to see how much it spreads out, we will first apply a thin film of Lycopodium Power (it looks a lot like chalk dust) onto the water surface. When a drop of oil is added, the powder will spread out circularly and all we need to do is measure the diameter of it. If we know how much oil is in the droplet, we can then determine the thickness of the slick and assume this is the size of one molecule.

        To measure the thickness of a single molecule. This gives us a "feel" for the scale of matter at the microscopic level of molecules and atoms.

Apparatus:
 

• 1        Tray
•  Sokution of Oleic Acid / Alcohol
• 1        Metre Stick
• Lycopodium Powder
• Eye-dropper
• Graduated Cyclinder, 10 ml

Procedure:
 

1. Before you do anything, you'll need to know how much oleic acid is in one drop. Take the given mixture and select one person in your lab group that will do the dropping. This is important that you get some consistency. If you have several people make drops, the sizes will probably vary. Use the eye dropper and count the number of drops to make 1 ml in your graduated cyclinder. The volume of one drop is merely:

Volume of 1 drop = 1 cm³ / # drops in 1 ml

You can do this a few times to confirm your value. Find the volume of one drop.

2. Your instructor has mixed the solution of 5 cm³ of oleic acid per 1000 cm³ of alcohol. Thus, each drop drop of solution contains 1 / 200th of the volume of a drop of oil. i.e.; the volume of oil in one drop is the volume of one drop divided by 200.

eg.  If  there are 50 drops in 1 ml, then each drop has a volume of 1 / 50 ml or 1 / 50 cm³. The amount of oleic acid (oil) in one drop is then:

1 / 50  X 1 / 200  = 1 / 10,000  =  .0001 cm³

3. Place the water in the tray, about 1 to 2 cm deep.  Lightly spread the powder over the water. This step requires a soft touch.

4.  Have your lab partner who does the drops actually drop one little drop onto the surface. The yellowish powder will recede rather quickly. Someone has to measure the diameter of the oil slick. Do this quickly as there may be some recoil, i.e.; the slick amy even oscillate.

5. The area of the slick is merely pr² so we can find the volume of the oil slick as thickness X area:

Volume = pr² X t

6.  Find the thickness of the molecule or basically of the oil slick. If several lab groups do this, take all the values and report the mean value. This allows you to estimate an uncertainty as well (as per the first lab we did together.)

7.  List LIMITATIONS to the experiment. (Yeah, there really are a few.)

8.  Record your data clearly and show your analysis.

9.  Report your RESULTS.

10.  SUMMARIZE the experiment and state your CONCLUSIONS

11..RECOMMENDATIONS??

12. Have a nice day!!