Question

Which colligative property is most suitable for determining molar mass?

• A. Relative lowering of vapour pressure
• B. Elevation in boiling point
• C. Depression in freezing point
• D. Osmotic pressure

Hint:

Among all colligative properties, \[ \boxed{\text{Osmotic pressure}} \] is preferred for determining the molar masses of proteins, polymers and other high molecular mass compounds because it can be measured accurately in dilute solutions.

Answer 1

The Correct Option is D

Concept: Colligative properties depend only on the number of solute particles present in a solution and not on their chemical nature. The four important colligative properties are:

• Relative lowering of vapour pressure

• Elevation in boiling point

• Depression in freezing point

• Osmotic pressure
Among these, osmotic pressure is the most accurate method for determining the molar masses of macromolecules such as proteins and polymers because it can be measured even in very dilute solutions. The osmotic pressure equation is \[ \boxed{\pi=\frac{nRT}{V}} \] or \[ \boxed{\pi=CRT} \] where \[ \pi=\text{Osmotic pressure}, \] \[ C=\text{Molar concentration}. \]

Step 1: Understand the advantages of osmotic pressure.
Osmotic pressure can be measured accurately at room temperature. It does not require heating or cooling of the solution. Therefore, decomposition of solute molecules is avoided.

Step 2: Compare with other colligative properties.
Relative lowering of vapour pressure is usually very small. Elevation in boiling point and depression in freezing point require comparatively concentrated solutions. Osmotic pressure can be measured even for dilute solutions and gives highly accurate results.

Step 3: Identify the correct option.
Hence, the most suitable colligative property for determining molar mass is \[ \boxed{\text{Osmotic pressure}.} \] Therefore, \[ \boxed{\textbf{Option (D)}} \] is the correct answer.