Question

Explain Raoult’s law. How would you distinguish between ideal and non-ideal solutions on its basis?

Hint:

Raoult’s law is crucial for understanding how solute and solvent interact in ideal solutions and how deviations arise in non-ideal solutions.

Answer 1

Step 1: Understand Raoult’s Law.
Raoult’s Law states that the partial vapor pressure of each volatile component in a solution is directly proportional to its mole fraction in the solution. The law is given by the equation: \[ P_A = X_A \cdot P_A^0 \] Where: - \( P_A \) is the partial vapor pressure of component A in the solution. - \( X_A \) is the mole fraction of component A in the solution. - \( P_A^0 \) is the vapor pressure of pure component A. Raoult’s law is valid for ideal solutions, where intermolecular forces between the solute and solvent are similar to those in the pure components.
Step 2: Ideal Solutions.
An ideal solution follows Raoult’s law exactly. The intermolecular forces between the solute and solvent are similar to the forces between the solute molecules and solvent molecules. As a result, the vapor pressure of the solution is a linear function of the mole fraction of each component. Examples: Benzene and Toluene.
Step 3: Non-ideal Solutions.
Non-ideal solutions deviate from Raoult’s law. These deviations occur because the intermolecular forces between the solute and solvent molecules are different from the forces in the pure components. In non-ideal solutions, the vapor pressure is either higher or lower than predicted by Raoult’s law. - Positive deviation: Occurs when the intermolecular forces between solute and solvent are weaker than those between the solute-solute and solvent-solvent molecules. - Negative deviation: Occurs when the intermolecular forces between solute and solvent are stronger than those between solute-solute and solvent-solvent molecules. Examples: Acetone and chloroform exhibit negative deviation, and alcohol and ether show positive deviation.
Final Answer: Raoult's law describes the vapor pressure of an ideal solution. Ideal solutions obey Raoult’s law strictly, while non-ideal solutions deviate from it due to differences in intermolecular forces.
Final Answer: \[ \boxed{\text{Raoult’s law describes the vapor pressure of ideal solutions, which obey it strictly, while non-ideal solutions deviate due to different intermolecular forces.}} \]