key term - Raoult's Law

Definition

Raoult's Law states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent present. This law highlights the relationship between the composition of a solution and the vapor pressure, which is crucial for understanding how solutes affect the properties of solvents and solutions.

5 Must Know Facts For Your Next Test

1. Raoult's Law applies primarily to ideal solutions, where the solute-solvent interactions are similar to solvent-solvent interactions.
2. As more solute is added to a solvent, the vapor pressure of the solvent decreases according to Raoult's Law.
3. The law can be used to calculate the boiling point elevation and freezing point depression of solutions, which are colligative properties.
4. For non-volatile solutes, Raoult's Law predicts that the total vapor pressure of a solution will be lower than that of the pure solvent.
5. Deviations from Raoult's Law occur in real solutions due to differences in intermolecular forces between solute and solvent molecules.

Review Questions

• How does Raoult's Law explain the change in vapor pressure when a non-volatile solute is added to a solvent?
  • Raoult's Law explains that when a non-volatile solute is added to a solvent, the mole fraction of the solvent decreases. Since vapor pressure is directly proportional to this mole fraction, the overall vapor pressure of the solution is reduced compared to that of the pure solvent. This occurs because the presence of solute particles hinders the escape of solvent molecules into the vapor phase.
• What are colligative properties and how are they related to Raoult's Law?
  • Colligative properties are physical properties of solutions that depend on the number of solute particles rather than their identity. Raoult's Law provides a foundational understanding for these properties, such as boiling point elevation and freezing point depression. According to Raoult's Law, as more solute is dissolved, the vapor pressure decreases, leading to changes in these colligative properties.
• Evaluate how deviations from Raoult's Law can inform us about molecular interactions in a solution.
  • Deviations from Raoult's Law indicate differences in molecular interactions within a solution. Positive deviations suggest that interactions between different molecules are weaker than those among similar molecules, leading to higher vapor pressures than expected. Conversely, negative deviations imply stronger interactions between solute and solvent molecules than between solvent molecules alone. Analyzing these deviations allows for insights into molecular compatibility and interaction strength within solutions.

Related terms

Vapor Pressure: The pressure exerted by a vapor in equilibrium with its liquid or solid form, indicating how easily a substance evaporates.

Mole Fraction: A way of expressing the concentration of a component in a mixture, defined as the ratio of the number of moles of that component to the total number of moles of all components.

Ideal Solution: A solution that follows Raoult's Law perfectly, where interactions between different molecules are similar to interactions between like molecules.