indirect method of measurement
Atoms and molecules are small. No, really. So small that you cannot see them with the unaided eye, which makes
their size all the more difficult to measure. However, using some simple materials and methods, we can get a
pretty reasonable approximation of the size of a molecule. You are going to use an indirect method of measurement again.
We are going to find the size of a molecule of oleic acid (fatty acid) found in olive oil. You learnt about these molecules in Biology class.
Oleic acid has the chemical formula C17H33COOH. The COOH part is the "head"of the molecule, the polar end.
A diagram of its structure is shown below:
The density of olive oil is smaller than 1 so oil floats on water.
Due to its large nonpolar portion, oleic acid is insoluble in water and, being less dense than water, it floats on the water surface.
The
polar –COOH end, however, is attracted to water molecules. So, a thin
coating of oleic acid on the surface of water results in
the molecules aligning themselves in an upright position with the polar ends oriented toward the water surface. See below.
If there is sufficient surface area for the water, the oleic acid will spread out and
will form a film one molecule thick. Such a layer is known as a monolayer.
The idea of this lab is to find the height (the thickness) of this monolayer.
Indeed the monolayer is approximately equal to the length of the molecule of oleic acid.
We suppose the layer of oil to be a cylinder of unknown height. The volume of oil dropped on the water is known, the area of the layer can be computed so we can find the height.
II) INTRODUCTION - MATHLet's check your Math skill first.
A) formula for the area of the base of a cylinder = A = ________ (given the radius R)
You will compute the area of the spread-out drop.
formula for the volume of a cylinder, given the area A and the height H = V = __________.
Can you find the formula for H, given A and given V ? H = ___/ _____
Example: if a cylinder has a diameter of 6cm, its radius R = _______ cm and its base's area = A = ______ cm2
If the volume of the cylinder is 10 cm3, can you find H ? H = ___/____ = ______ cm
B) Also 1 milliliter (mL) = 1,000 microliter (μL)
and you remember 1 mL is the same as 1 cm3 (same volume, same space)
so 1 microliter = _________ milliliter or 1 mL = _______ 1 μl (use proportion if you want)
so 5 μL = _________ mL (or cm3) or 5 μL = ______ cm3 (ask DL to check)
III - PROCEDURE
Be very careful. The micro pipette is very fragile and is used by seniors. Don't play with them, be very gentle, don't push hard on the cap, don't force it. If you damage one pipette you get a 0 on the lab and you will have to replace it.STEP1 - Get a micro pipette from your instructor and set it so it will suck just 2 microliter. Ask your instructor for help.
STEP2 - get a container in glass with a large base. Place the container on a black table. With a large beaker
fill the container with water. It should be only half full. wait for the water to be perfectly still. don't move the table. Be careful.
STEP3 - your partner should have good eyes and a ruler in centimeters. She/He should be ready to use it.
STEP4 - From DL test tube you need to extract just 2 μL of olive oil. DL will show you.
STEP5 - drop the oil in your container. You should see the oil quickly spreading out. It is hard to see the spread-out but look for the circular colors. With the ruler, measure the diameter of the spread-out of the drop in cm.
D = ___________ cm DON'T ROUND. FROM NOW ON keep as much as decimals as possible.
Find the radius R = _________ cm (you should have 1 decimal for the mm)
PART IV - ANALYSIS
1) don't round anything. Keep as much decimals as possible
Find the area A of the spread-out = ___________ cm2
2) you have the volume of the drop V = 2 μL = ______ mL
(hint: convert to mL, divide by 1000 )
but mL and cm3 is the same thing so V = 2 μL = ______ cm3
3) You have the volume, you have the area you can find the height H in cm (really small number)
H = V/ A = __________ cm convert to μm (make a table, check with DL for the factor, multiply by ..)
H = _________ μm
4) In our first class you learnt that a molecule is in the nanometer range =10-9 m range = 0.001 μm range
Is it consistent with what you found?
Search on the Internet the actual size of a molecule of oleic acid = _________ μm.
Is is consistent ?
5) Use now the actual size of a oleic acid molecule.
Search the size of a water molecule (Internet). size = _________μm
is it smaller ? discuss.
PART V - GOING FURTHER - connection with your Biology class
1) What kind of fat is olive oil ?
2) Why is olive oil a liquid at room temperature ? detail your answer.
3) Why does the head of the molecule touch the water and not the tail ?
4) Is fat found in animals solid or liquid ? why ?
5) An old riddle asks, how many angels can dance on the head of a pin? If we assume that an angel is the same
size as an oleic acid molecule, and the circular head of a pin is 2mm across, how many angels will fit?
(hint: 2mm = 2,000 μm)
6) Find the density of olive oil (Internet). density = ________ g/cm3 . Find the mass of your drop.
Use μg . m = _______ μg. (1g = 1,000,000 μg)
7) Some scientists were able to find the diet of gladiators by using a indirect method of measurement called
atomic spectroscopy. They used a tiny piece of a bone of a dead gladiator , crushed to a power and
were able to tell they ate mostly beans and barley. The idea was to make them gain weight so the layer of fat would protect their organs. Describe this method (atomic spectroscopy)