Question

State Raoult’s Law for volatile solutes and explain its deviations.

Hint:

Positive deviation = weaker intermolecular forces and higher vapor pressure; Negative deviation = stronger forces and lower vapor pressure.

Answer 1

Concept: Raoult’s Law describes the vapor pressure behavior of ideal solutions containing volatile components. It relates the partial vapor pressure of each component to its mole fraction in the liquid phase.
Step 1: Statement of Raoult’s Law. For a solution of volatile liquids, the partial vapor pressure of each component is proportional to its mole fraction in the solution. \[ p_A = x_A p_A^\circ \quad \text{and} \quad p_B = x_B p_B^\circ \] where $p_A^\circ$ and $p_B^\circ$ are vapor pressures of pure components and $x_A, x_B$ are their mole fractions. Total vapor pressure: \[ P = p_A + p_B \]
Step 2: Ideal solution behavior. Raoult’s Law is strictly followed by ideal solutions where:

• Intermolecular forces between unlike molecules are similar to those between like molecules
• No heat change on mixing
• Volume of solution is additive

Step 3: Positive deviation from Raoult’s Law. Occurs when intermolecular attractions between unlike molecules are weaker than those in pure components.

• Vapor pressure becomes higher than predicted
• Boiling point decreases
• Example: Ethanol–acetone mixture

Step 4: Negative deviation from Raoult’s Law. Occurs when intermolecular attractions between unlike molecules are stronger than those in pure components.

• Vapor pressure becomes lower than expected
• Boiling point increases
• Example: Chloroform–acetone mixture

Conclusion: Raoult’s Law explains vapor pressure behavior in ideal solutions, while deviations arise due to differences in intermolecular interactions, leading to positive or negative deviations.