Question

What is molal elevation constant? How is it related to molarity of a solution?

Hint:

Molal elevation constant (\(K_b\)) and molal depression constant (\(K_f\)) are examples of colligative property constants.

Answer 1

Step 1: Definition.
Molal elevation constant (also called ebullioscopic constant, \(K_b\)) is defined as the elevation in boiling point of a solvent when 1 mole of a non-volatile solute is dissolved in 1 kg of the solvent.
\[ \Delta T_b = K_b \cdot m \] where, \(\Delta T_b\) = elevation in boiling point,
\(m\) = molality of the solution.
Step 2: Explanation.
It depends only on the nature of the solvent, not on the solute. The constant \(K_b\) has units of K·kg·mol\(^{-1}\).
Step 3: Relation with molality.
Since the formula involves molality, the rise in boiling point of a solution is directly proportional to its molality.
\[ \Delta T_b \propto m \] Step 4: Distinction from molarity.
- Molality is defined per kg of solvent and is temperature-independent.
- Molarity is defined per liter of solution and is temperature-dependent.
Thus, molal elevation constant is connected with molality of the solution, not with molarity.