Oleic Acid Lab
Molecules that are repelled by water are called hydrophobic. Molecules that are attracted to water are called hydrophilic. Cooking oil is hydrophobic; it won't mix with water. Some molecules have one end that is hydrophobic and one end that is hydrophilic. There are such molecules in the cells in your body. They are used to take hydrophobic nutrients into the cell that is mostly water. Soaps are this way also so that they can dissolve both hydrophobic and hydrophilic substances and be washed away by water.
Oleic acid is a substance with one hydrophobic and one hydrophilic end. When a small amount of oleic acid is placed on the surface of water, it stands on end with the hydrophilic end towards the water and the hydrophobic end away. If you could see them, they would look like fans at a crowded concert.
In this lab, we will find the length of one oleic acid molecule by spreading a small amount over the surface of water and measuring the diameter of the circle. The oleic acid spreads itself into a one-molecule thick layer in the shape of a VERY flat cylinder.
Oleic acid layer
Water layer
If we know the volume of the drop of oleic acid and the diameter of the layer, the height of the layer (length of the molecule) can be found. Recall that the equation for the volume of a cylinder is pr2h.
Pre-lab questions:
1. Measure the height and diameter of a soda can in cm. Calculate the volume of the can.
2. What are some sources of error in your measurements?
3. How much empty space must be in the can according to the volume of soda in the can? Recall that a mL and a cm3 are the same thing.
Procedure:
1. First, you must figure out the volume of a drop of water (the liquid that you will be given is very similar to water). You will find some calibrated pipettes (plastic droppers with .25, .5, .75, and 1 mL markings). When the level of liquid is at the line at the base of the bulb, there is 1.0 mL of liquid. Be sure that there are no air bubbles. A good way to fill it exactly is to fill it slightly less than 1 mL and then let a couple of air bubbles out and liquid replace it. Repeat this until filled exactly to 1.0 mL.
2. Now count how many drops it takes to completely expel the liquid. Be careful at the end that some apparent bubbles will be mostly air. These can count as a half-drop. Repeat this procedure three times and record the results in your data chart.
3. Your teacher should have come around already and sprinkled some powder over the water in you pizza pan. Do not disturb this water. It must be still to perform the experiment.
4. Your teacher will come by and put a drop of oleic acid solution in the center of your pan. The oleic acid will push the powder out of the way forming a circle. Measure the diameter of the circle in three different places and record the result.
5. Clean up your area and wash the pizza pan well with soap to get all of the oleic acid off.
Data chart:
Drops/mL ________ drops ________ drops ________ drops Average ___________ drops
Circle Diameter (cm)
1. ___________ cm
2. ___________ cm
3. ___________ cm
Average___________ cm
Calculations:
1. Calculate the volume of one drop of liquid. ___________ mL
2. The oleic acid is not pure. .8 drops of oleic acid were added to 96 drops of alcohol. What percent of the volume is oleic acid? Now multiply the volume of one drop and the percent of oleic acid (converted to a decimal) and find what volume of oleic acid is in one drop.
_____________% oleic acid _____________ volume oleic acid/drop
3. The equation for the volume of a cylinder is V=pr2h. Solve this equation for h.
4. Now use the equation for h, your average diameter (converted to a radius) and your answer to #2 (volume) to find the height of an oleic acid molecule.
Post-lab questions:
1. Your teacher will tell you the accepted value for the length of an oleic acid molecule. Find your percent error.
2. Why did we use such a small amount of oleic acid?
3. If your measurements for the diameter of the circle were too large, how would that affect your answer? Explain
| Questions? Comments?? |
|
 |