Raoult's Law & Solutions
Vapor Pressure, Ideal/Non-Ideal Solutions & Azeotropes
1. Raoult's Law (Volatile Solute)
Statement: For a solution of volatile liquids, the partial vapor pressure of each component in the solution is directly proportional to its mole fraction present in the solution.
For component A: $p_A \propto \chi_A \Rightarrow p_A = p_A^\circ \chi_A$
For component B: $p_B \propto \chi_B \Rightarrow p_B = p_B^\circ \chi_B$
$$ P_{total} = p_A + p_B = p_A^\circ \chi_A + p_B^\circ \chi_B $$
Where $p^\circ$ represents vapor pressure of pure component.
2. Raoult's Law (Non-Volatile Solute)
When a non-volatile solute (e.g., sugar, salt) is added to a volatile solvent, the vapor pressure of the solvent decreases. This is a Colligative Property.
Relative Lowering of Vapor Pressure:
$$ \frac{P_A^\circ - P_S}{P_A^\circ} = \chi_{solute} $$
Where $P_S$ is vapor pressure of solution and $\chi_{solute}$ is mole fraction of solute.
3. Ideal vs Non-Ideal Solutions
| Ideal Solutions | Non-Ideal (Positive Deviation) | Non-Ideal (Negative Deviation) |
|---|---|---|
| Obey Raoult's Law ($P_A = P_A^\circ \chi_A$) | $P_A > P_A^\circ \chi_A$ (Vapor pressure higher) | $P_A < P_A^\circ \chi_A$ (Vapor pressure lower) |
| $\Delta H_{mix} = 0$, $\Delta V_{mix} = 0$ | $\Delta H_{mix} > 0$ (Endothermic), $\Delta V_{mix} > 0$ | $\Delta H_{mix} < 0$ (Exothermic), $\Delta V_{mix} < 0$ |
| A-B interactions $\approx$ A-A or B-B | A-B interactions weaker than A-A/B-B | A-B interactions stronger than A-A/B-B |
| Ex: Benzene + Toluene | Ex: Ethanol + Acetone | Ex: Chloroform + Acetone |
4. Azeotropes
Binary mixtures having the same composition in liquid and vapor phase and boil at a constant temperature.
- Minimum Boiling Azeotrope: Shows large positive deviation (e.g., 95% Ethanol + Water).
- Maximum Boiling Azeotrope: Shows large negative deviation (e.g., 68% $HNO_3$ + Water).
Practice Quiz
Test your knowledge on Raoult's Law.
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