Small amounts of a nonvolatile, nonelectrolyte solute and a...

Small amounts of a nonvolatile, nonelectrolyte solute and a volatile solute are each dissolved in separate beakers containing $1 \mathrm{~kg}$ of water. If the number of moles of each solute is equal: a. Which solution will have the higher vapor pressure? b. Which solution will boil at a higher temperature?

Small amounts of a nonvolatile, nonelectrolyte solute and a volatile solute are each dissolved in separate beakers containing $1 \mathrm{~kg}$ of water. If the number of moles of each solute is equal:
a. Which solution will have the higher vapor pressure?
b. Which solution will boil at a higher temperature?

Key Concepts

Colligative Properties

Colligative properties are characteristics of solutions that depend solely on the number of solute particles and not on their identity. This includes phenomena such as vapor pressure lowering and boiling point elevation, meaning that even small amounts of solute can significantly alter these physical properties if the number of particles present is substantial enough.

Raoult's Law

Raoult's Law states that the vapor pressure of a solvent in an ideal solution is proportional to its mole fraction in that solution. This concept is central to understanding how the addition of a nonvolatile solute reduces the solvent's vapor pressure, as the decrease is directly related to the dilution of the solvent's mole fraction.

Vapor Pressure Lowering

Vapor pressure lowering occurs when a solute is dissolved in a solvent, reducing the number of solvent molecules available at the surface to escape into the vapor phase. This is particularly evident with nonvolatile solutes, which do not contribute significantly to the vapor phase, thereby causing a decrease in the overall vapor pressure of the solution.

Boiling Point Elevation

Boiling point elevation is the phenomenon where a solution’s boiling point is higher than that of the pure solvent. Since boiling occurs when the vapor pressure equals the external pressure, a lowered vapor pressure—due to the addition of a nonvolatile solute—necessitates a higher temperature to reach boiling, thereby elevating the boiling point of the solution.

Volatile versus Nonvolatile Solutes

Volatile solutes contribute their own vapor pressure to the overall pressure of a solution, whereas nonvolatile solutes do not. As a result, while both types affect colligative properties, a volatile solute can raise the total vapor pressure through its own evaporation, contrasting with a nonvolatile solute which primarily affects the solvent's properties by reducing its mole fraction.

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