Question

Define Osmosis. How will you determine molar mass of non-volatile solute by elevation of boiling point?

Hint:

Use precise temperature measurements and known \( K_b \) values for accurate molar mass determination.

Answer 1

Definition: Osmosis is the spontaneous movement of solvent molecules through a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration, tending to equalize concentrations.
Determination of Molar Mass:
The elevation of boiling point (\( \Delta T_b \)) for a non-volatile solute is given by: \[ \Delta T_b = K_b \cdot m, \] where \( K_b \) is the molal boiling point elevation constant, and \( m \) is molality (\( \text{mol kg}^{-1} \)).
Step 1: Measure the boiling point of the pure solvent (\( T_0 \)) and the solution (\( T \)). Calculate \( \Delta T_b = T - T_0 \).

Step 2: Molality \( m = \frac{n_B}{w_A} \), where \( n_B = \frac{w_B}{M_B} \) (moles of solute, \( w_B \) = mass of solute, \( M_B \) = molar mass of solute), and \( w_A \) = mass of solvent in kg.
\[ \Delta T_b = K_b \cdot \frac{w_B}{M_B \cdot w_A}. \] \[ M_B = \frac{K_b \cdot w_B}{\Delta T_b \cdot w_A}. \]
Step 3: Use known \( K_b \), measured \( \Delta T_b \), \( w_B \), and \( w_A \) to calculate \( M_B \).