Measuring the Length of a Molecule
Publication No. 11888
Is it possible to measure the length of a single molecule using a quick and easy procedure? Yes! Irving Langmuir received the Nobel Prize in chemistry in 1932 for his work on monomolecular films, which are films that are assumed to have a thickness of one molecule. By using a method similar to his, the approximate length of a single oleic acid molecule can be determined.
Ethyl alcohol is flammable and a dangerous fire risk. Lycopodium is a highly flammable powder and a possible allergen. Avoid contact with skin. Wear chemical splash goggles, a chemical-resistant apron and chemical-resistant gloves. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Flush the oleic acid/ethyl alcohol/water mixture in the tray down the drain according to Flinn Suggested Disposal Method #26b.
Student Worksheet PDF
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesDeveloping and using models
Disciplinary Core IdeasMS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
Crosscutting ConceptsScale, proportion, and quantity
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
Plugging the above sample data into the formula for the volume of a cylinder, a length of 3.3 x 10–7 cm is obtained for the length of the oleic acid molecule. In general, the length will turn out to be in the neighborhood of 10–7 cm.
V = π r2 h
Trial 1 = 26 cm Trial 2 = 30 cm Trial 3 = 28 cm
Radius of oleic acid circle:
Trial 1 = 13 cm Trial 2 = 15 cm Trial 3 = 14 cm
Average radius = 14 cm
Answers to Questions
The circle of single-layer oleic molecules can be compared to a cylinder with a very small height. Since the molecules stick straight up in the air, forming the “wall” of the cylinder, the height of the cylinder is actually equal to the length of one oleic acid molecule.
Oleic acid, C18H34O2, is a fatty acid molecule that has two distinct parts. One end is a nonpolar, hydrophobic (water-fearing) carbon chain, resembling a “tail.”. The other end is a polar, hydrophilic (water-loving) “head.” Fatty acid molecules are commonly simplified using “head and “tail” drawings, such as the one in Figure 1.
Oleic acid is soluble in many organic solvents, such as ethyl alcohol, but not in water. When placed in the polar solvent water, it tends to form a monolayer with the polar “head” sticking down towards the surface of the water and the nonpolar “tail” sticking straight up away from the surface of the water (see Figure 2).
When the drop of oleic acid/ethyl alcohol solution is placed onto the surface of the water, the alcohol and the oleic acid separate. The ethyl alcohol is soluble in water, and it dissolves in the water as soon as the drop is placed on the surface of the water. But, because the oleic acid is not soluble in water, it does not dissolve in the water and instead spreads out on top of the water to form a layer that is roughly circular in shape. The length of the molecule is calculated by assuming that this “circle” is actually a cylinder with a very small height. The height of the cylinder is equal to the length of the oleic acid molecule.
Bilash, B; Gross, G.; Koob, J. A Demo A Day™, Volume 2—Another Year of Chemical Demonstrations; Flinn Scientific Inc.: Batavia, IL, 1996; p 5.