Question

Define Raoult's Law and explain it.

Hint:

Raoult's Law applies ideally to dilute solutions. For non-ideal solutions, deviations (positive/negative) occur due to intermolecular forces.

Answer 1

Definition:
Raoult's Law states that the relative lowering of vapour pressure of a solution is equal to the mole fraction of the solute.
For a solution containing a non-volatile solute: \[ p_A = p_A^\circ \cdot x_A \] where: - \(p_A\) = vapour pressure of solvent in solution,
- \(p_A^\circ\) = vapour pressure of pure solvent,
- \(x_A\) = mole fraction of solvent.
Explanation:
Adding a non-volatile solute decreases the mole fraction of solvent. As vapour pressure is directly proportional to solvent mole fraction, the vapour pressure decreases. The relative lowering of vapour pressure is: \[ \frac{p_A^\circ - p_A}{p_A^\circ} = x_B \] where \(x_B\) is the mole fraction of solute.
Thus, Raoult's Law explains colligative properties like relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure.