The correct answer is Option (3) → Osmotic pressure
While all four properties listed are technically colligative and can be used to determine molar mass, osmotic pressure is the preferred and most practical method for biomolecules (like proteins, DNA, and enzymes) for several critical reasons:
Why Osmotic Pressure is Preferred
- Temperature Sensitivity: Biomolecules are often unstable and can denature (break down) at high temperatures. Methods like "Elevation in boiling point" would require heating the solution, which could destroy the sample. Osmotic pressure is measured at room temperature.
- Magnitude of Measurement: Biomolecules usually have very high molar masses, meaning even a significant mass of the substance results in a very low number of moles. In such dilute solutions, the changes in freezing point or boiling point are too small to measure accurately (0.001 K or less). However, the osmotic pressure remains large and easily measurable even for very dilute solutions.
- Solubility: Many polymers and biomolecules have poor solubility in common solvents. Osmotic pressure works effectively even with these small concentrations.
- Use of Molarity: This method uses molarity (M) instead of molality (m), which is often more convenient for liquid-phase biological research.
The Calculation Formula
The molar mass ($M_B$) can be calculated using the van't Hoff equation:
$π = \frac{w_BRT}{M_BV}$
Where:
- $π$ = Osmotic pressure
- $w_B$ = Mass of the solute
- $R$ = Gas constant
- $T$ = Temperature in Kelvin
- $V$ = Volume of the solution
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