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## Measuring the Size of a Molecule

### Preparing for the Lab

##### Lab Objective

Using tools readily available in a high school chemistry lab you will measure the length and the mass of a molecule of the fatty acid from olive oil, known as oleic acid (or cis-9-octadecenoic acid). Oleic acid’s formula is C18H34O2 and its structure is shown below. Our goals:

1. To determine the length (long dimension) and width (short dimension) of oleic acid molecules
2. To determine the number of individual molecules in a single drop of pure oleic acid
3. To determine the mass of a single molecule

Briefly, you will use a solution of oleic acid in methanol to make a slick of oleic acid molecules on the surface of a tray full of water. You will make the slick visible by spreading a very fine powder on the surface before dropping the oleic acid onto the surface. You will find the size, number, and mass of individual oleic acid molecules by understanding and using simple mathematics around density, volume, and concentration. Your teacher will demonstrate the procedure for you. ##### Calculations

For this lab you will need to be able to do the following calculations:

1. Find the volume of a small drop of liquid by counting how many drops it takes to fill up 1 mL.
2. Use the concentration of oleic acid (3.06 × 10-3 g/cm3) and the density of oleic acid (0.890 g/cm3) to determine the mass and volume of pure oleic acid in one drop of the methanol solution.
3. Using the volume formula relating volume, area, and height, find the height of the spot of oleic acid. Use the volume of pure oleic acid determined in the previous step. Since the oil spreads out to an area only one molecule thick, this is the length of the molecule. Report your answer in cm and nm.
4. Calculate the width of the molecule in cm and nm. Assume that the molecule is 1/4 as wide as it is tall.
5. Assuming that the base of the molecule is a square, find the area of the base of the molecule in cm2. Use that result to calculate the number of molecules in the spot of oleic acid.
6. Calculate the mass of one molecule using the mass of oleic acid in one drop (the mass of oleic acid in the spot) and the number of molecules.
7. Find the precision your measurements of the length, width and mass of oleic acid molecules. Express the precision as a percent error by dividing the ± amount by the size of the measurement.

 Oleic Acid C18H34O2 282.47 g/mol Length 1.97 nm Width 0.5 nm Mass 4.69 × 10-22 g
These data are provided in order for you to decide whether your experimental results are accurate. You can calculate a percent error for accuracy by using the following formula:
```          | literature value – experimental value |
% error = ———————————————————————————————————————— × 100%
literature value
```
Source for these data and the density of oleic acid: www.chemexper.com, search on ‘oleic acid’.
Idea for and design of this lab is courtesy of Dr. Richard Lewis, Scarborough High School.

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##### Exercises

Do the following exercises as practice for the calculations you will do for this lab.

Volume of a Single Drop
Given the following number of drops needed to exactly fill 1.0 mL determine the volume of one drop in mL.
1. 16
2. 25
3. 37
Mass & Volume of Oleic Acid
You are given the volume of a solution of oleic acid in methanol. The concentration of that solution is 3.6 × 10-3 g/cm3. The density of oleic acid is 0.890 g/cm3. Find the mass and volume of oleic acid in the given volume of solution as if there were no methanol solvent.
1. 0.063 mL
2. 0.040 mL
3. 0.027 mL
Length of a Molecule
Given the volume and area of a spot of oleic acid which has formed on the surface of a tray full of water you can calculate the length of a molecule. Sinc the spot is only one molecule thick, the height of the spot is the length of the molecule. Use the volume formula V = A × H and the data given in each problem below to calculate the length of a molecule. Report the answer in cm and nm.
1. V = 2.27 × 10-4 cm3; A = 1.15 × 103 cm2
2. V = 1.12 × 10-4 cm3; A = 5.47 × 102 cm2

3. V = 1.28 × 10-4 cm3; A = 6.79 × 102 cm2
Width of a Molecule
Assuming that oleic acid molecules are 1/4 as wide as they are long calculate the width of an oleic acid molecules using your results from the previous three problems. Express the result in both cm and nm.
1. for the result from number 7
2. for the result from number 8
3. for the result from number 9
Number of Molecules
Assuming that the base of the molecule is a square, find the area of the base of the molecule in cm2. Use that result to caculate the number of molecules in a spot of oleic acid. Use the width you calculated in numbers 10 - 11 for the width of an oleic acid molecule. Use the corresponding area from numbers 7 - 9.
1. Width from 10:
Area from 7:
2. Width from 11:
Area from 8:
3. Width from 12:
Area from 9:

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Mass of One Molecule
Calculate the mass of one molecule in grams using the mass of oleic acid in one drop (the mass of oleic acid in the spot) and the number of molecules. Use the data from previous problems for your work. (Calculate Mass using Volume from 7 - 9; number of molecules from 13 - 15).
1. m =
2. m =
3. m =
Data Tables and Precision

Create a data table that will show the results of each of the three trials you have worked out in these exercises so far. Sketch out what columns you will use and present your ideas to your partners and the teacher. What is important enough to include? What are the raw data here? The results? How will you display averages, ranges and ± precision? Create your table below.

Calculate the range and precision of each measurement. Calculate the precision percent experimental error by dividing the ± amount by the average measurement it belongs to. Also, calculate the accuracy percent experimental error by comparing your results to the literature values given on the first page of this handout. A formula is there for you to use as well. Be sure to leave room for the results of these calculations in your table.

Find the lab procedure here.
Last updated: Nov 04, 2007 Home