Understanding Vapor Pressure and Colligative Properties
* Vapor Pressure: The pressure exerted by the vapor of a liquid in equilibrium with its condensed phase.
* Colligative Properties: Properties of solutions that depend only on the number of solute particles present, not their identity.
* Vapor Pressure Lowering: One of the colligative properties. Adding a non-volatile solute to a solvent lowers the vapor pressure of the solvent.
Factors Affecting Vapor Pressure Lowering
1. Nature of the Solute: Non-volatile solutes (those that don't readily evaporate) cause the greatest vapor pressure lowering.
2. Concentration of the Solute: The higher the concentration of solute particles, the greater the lowering of vapor pressure.
Analyzing the Solutions
* Alcohol in Water: Alcohol is somewhat volatile, so its vapor pressure lowering will be less pronounced compared to a completely non-volatile solute.
* Sugar in Benzene: Sugar is non-volatile, and benzene is a volatile solvent. This combination will result in significant vapor pressure lowering.
* NaCl in Water: NaCl is ionic and dissociates into two particles (Na+ and Cl-) when dissolved in water. This creates a higher concentration of solute particles, leading to a greater lowering of vapor pressure compared to a non-ionic solute at the same molarity.
* Pure: A pure substance has no solute, so its vapor pressure will be the highest.
Conclusion
The solution with the lowest vapor pressure would be NaCl in water because:
1. NaCl is a non-volatile solute.
2. NaCl dissociates into two ions per formula unit, increasing the number of solute particles.
Important Note: The solutions are all 1.00 m (molal), which means they contain 1 mole of solute per kilogram of solvent. This allows us to compare the solutions based solely on the nature of the solute and its tendency to lower vapor pressure.
