Determination of Molecular Masses — Definition

Definition

Imagine you have a pure liquid, like water. Now, if you dissolve something in it, especially something that doesn't easily evaporate (a non-volatile solute), the properties of that water change. These changes are quite special because they don't depend on what you dissolved (e.

g., sugar or salt), but rather on how many particles of that substance you dissolved. These special properties are called 'colligative properties'. There are four main ones: the vapor pressure of the water goes down, its boiling point goes up, its freezing point goes down, and a new pressure called 'osmotic pressure' develops if the solution is separated from pure solvent by a semi-permeable membrane.

The exciting part for us, as NEET aspirants, is that we can use these changes to figure out the molecular mass of the unknown substance we dissolved. Think of it like this: if you know how much the boiling point went up, or how much the freezing point went down, or what the osmotic pressure is, and you know the amount of solvent you used, you can work backward to calculate the number of solute particles present.

Since molecular mass is essentially the mass of one mole of particles, if we know the total mass of the solute we added and the number of moles (from the colligative property measurement), we can easily find the molecular mass.

Among the four colligative properties, osmotic pressure is particularly powerful for determining the molecular masses of very large molecules, like proteins or synthetic polymers. Why? Because the changes in vapor pressure, boiling point, and freezing point are often very tiny and hard to measure accurately when dealing with such large molecules, as their molar concentrations are typically very low.

However, osmotic pressure produces a much larger, more easily measurable effect even at these low concentrations, and it can be measured at room temperature, which is crucial for temperature-sensitive biological samples.

So, by carefully measuring the osmotic pressure of a solution containing an unknown macromolecule, we can precisely determine its molecular mass, which is a fundamental piece of information in chemistry and biology.