Question

Cottrell’s method is used for the measurement of:

• A. Depression of freezing‐point
• B. Elevation of boiling‐point
• C. Lowering of vapour pressure
• D. Osmotic pressure

Answer 1

The Correct Option is B

Cottrell's method is commonly used in Pharmaceutical Chemistry for the measurement of the elevation of boiling-point, which is one of the colligative properties of solutions. 

Let's explore how Cottrell's method works and why it is used specifically for measuring the elevation of boiling-point:

1. Elevation of Boiling-Point: When a non-volatile solute is added to a solvent, the boiling point of the solution increases compared to the pure solvent. This is known as the elevation of boiling-point and is a colligative property—it depends only on the number of solute particles, not their identity.
2. Cottrell Method: This method involves measuring the boiling point of the solution and comparing it to the boiling point of the pure solvent to determine the elevation. The rise in boiling point, also known as boiling point elevation, is calculated using the formula: \(\Delta T_b = i \cdot K_b \cdot m\), where:
   • \(\Delta T_b\) is the boiling point elevation.
   • \(i\) is the van't Hoff factor (number of particles the solute splits into, for non-electrolytes it is 1).
   • \(K_b\) is the ebullioscopic constant of the solvent.
   • \(m\) is the molal concentration of the solution.
3. Why Not the Other Options?
   • Depression of Freezing-Point: This is another colligative property but involves the lowering of the freezing-point, not boiling-point elevation.
   • Lowering of Vapour Pressure: While related to boiling point, Cottrell’s method specifically measures the increase in boiling point, not the decrease in vapor pressure.
   • Osmotic Pressure: This property measures the tendency of solvent molecules to pass through a semipermeable membrane, unrelated to boiling point.

Therefore, Cottrell’s method is specifically used for the measurement of the elevation of boiling-point.