2. Experiment: Estimating the Size of Oleic Acid Molecules Using the Oil Film Method

Concept Explanation:
The oil film method is a classic experiment used to estimate the size of a molecule, specifically the length of an oleic acid molecule. The method involves spreading a small known quantity of oleic acid on the surface of water to form a monolayer, then measuring the area it covers.

Procedure:
1. Preparation: A small amount of oleic acid is dissolved in a volatile solvent (e.g., alcohol) to create a dilute solution.
2. Spreading the Film: A single drop of the oleic acid solution is carefully placed on the surface of the water in a shallow dish, and the solvent is allowed to evaporate, leaving behind a thin film of oleic acid molecules.
3. Measurement: The area of the film is measured. Since the film is assumed to be one molecule thick, the volume of oleic acid used divided by the area gives an estimate of the thickness of the molecule, which corresponds to the length of the oleic acid molecule.

Example Problem:

If 0.1 mL of oleic acid solution contains 0.001 mL of pure oleic acid and it spreads to cover an area of 20 cm² on the water surface, estimate the length of an oleic acid molecule.

Solution:

The thickness of the film (which corresponds to the length of the molecule) is given by the volume of oleic acid divided by the area:
Thickness = Volume of Oleic Acid / Area = 0.001 mL / 20 cm² = 0.00005 cm
So, the length of an oleic acid molecule is approximately 0.00005 cm or 5 × 10⁻⁸ m.