From the colligative properties of solution which one is the best method for the determination of mol.wt of proteins & polymers:

Options

A. Osmotic pressure
B. Lowering in V.P
C. Lowering is freezing point
D. Elevation in B.Pt

Answer

To determine the best method for calculating the molecular weight of proteins and polymers using colligative properties, we will analyze the options provided.

### Step-by-Step Solution:

1. **Understanding Colligative Properties**:
Colligative properties depend on the number of solute particles in a solution and include osmotic pressure, vapor pressure lowering, freezing point depression, and boiling point elevation.

2. **Analyzing the Options**:
- **A. Osmotic Pressure**: This property is defined by the formula:
\[ \Pi = iCRT \]
where \(\Pi\) is the osmotic pressure, \(i\) is the van 't Hoff factor, \(C\) is the concentration of the solute, \(R\) is the gas constant, and \(T\) is the temperature in Kelvin.
- **B. Lowering in Vapor Pressure**: This method is less effective for large molecules like proteins and polymers because it is more sensitive to small concentrations.
- **C. Lowering in Freezing Point**: While it can be used, it is not as straightforward for large macromolecules.
- **D. Elevation in Boiling Point**: Similar to freezing point depression, it is not as effective for determining molecular weights of large molecules.

3. **Why Osmotic Pressure is Best**:
- The osmotic pressure method allows for direct measurement of the concentration of solute particles in solution.
- For proteins and polymers, which are large molecules, the osmotic pressure can be easily measured, and the relationship between osmotic pressure and molecular weight is well-defined.
- By rearranging the formula for osmotic pressure, we can derive the molecular weight:
\[ M = \frac{iWRT}{\Pi V} \]
where \(W\) is the mass of the solute and \(V\) is the volume of the solution.

4. **Conclusion**:
Given the ease of measurement and the direct relationship to molecular weight, the best method for determining the molecular weight of proteins and polymers is **A. Osmotic Pressure**.

### Final Answer:
**A. Osmotic Pressure**