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Colligative Properties of Solutions

Key Concepts

  • Colligative properties of solutions depend on the concentration of solute particles but NOT on their identity.

  • Colligative properties depend on the lowering of the escaping tendency of solvent particles by the addition of solute particles.

  • Colligative properties include:
  1. vapor pressure lowering

  2. boiling-point elevation

  3. freezing-point depression

  4. osmotic pressure

Vapor Pressure Lowering

  • The escaping tendency of a solvent is measured by its vapor pressure.

  • Vapor pressure measures the concentration of solvent molecules in the gas phase.

  • Adding a nonvolatile solute lowers the vapor pressure of the solvent since a smaller proportion of the molecules at the surface of the solution are solvent molecules, fewer solvent molecules can escape from the solution compared to the pure solvent.

  • The quantitative relationship between vapor pressure lowering and concentration in an ideal solution is stated in Raoult's Law.

Boiling-Point Elevation

  • A liquid boils at the temperature at which its vapor pressure equals atmospheric pressure.

  • The presence of a nonvolatile solute lowers the vapor pressure of a solution so it is necessary to heat the solution to a higher temperature in order for it to boil.

  • The amount by which the boiling point is raised is known as the boiling point elevation.

  • The boiling-point elevation is proportional to the concentration of solute particles expressed as moles of solute per kilogram of solvent.

Freezing-Point Depression

  • The presence of a nonvolatile solute lowers the freezing point of a solvent.

  • In order to freeze the solvent, it must be cooled to a lower temperature in order to compensate for its lower escaping tendency.

  • The amount by which the freezing point is lowered is known as the freezing point depression.

  • The freezing-point depression is proportional to the concentration of solute particles expressed as moles of solute per kilogram of solvent.

Osmotic Pressure

  • When two liquids, such as a solvent and a solution, are separated by a semipermeable membrane that allows only solvent molecules to pass through, then there is a net transfer of solvent molecules from the solvent to the solution. This process is called osmosis.

  • Osmosis can be stopped by applying pressure to compensate for the difference in escaping tendencies. The pressure required to stop osmosis is called osmotic pressure.

  • In dilute solutions, osmotic pressure is directly proportional to the molarity of the solution and its temperature in Kelvin.

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