Colligative Properties

• Nonelectrolyte


  Compound that does not ionize in solution at all and will not conduct electricity, such as sugar and ethanol
• Electrolyte


  Substance that forms when dissolved in water, which is then able to conduct a current (conductivity). Strong electrolytes are considered to be completely ionized or dissociated in water, meaning they are soluble. Weak electrolytes are ionic compounds that are not completely dissociated or ionized in water.
• Strong electrolytes have greater effect on the properties of solution
• Colligative properties


  Depend only on the number of solute particles present (concentration), not on the identity of the solute particles. They include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure.

  
• Vapor pressure lowering


  • Nonvolatile solutes reduce the ability of surface solvent molecules to escape the liquid, therefore vapor pressure is lowered. The amount of vapor pressure lowering depends upon the number of solute particles.

  
• Boiling point elevation


  • Due to the lowering of the vapor pressure of the solvent when a solution forms, a solution will require a higher temperature than the pure solvent to reach its boiling point.

  
• Freezing point depression


  • Presence of a solute interferes with solid formation, requires more kinetic energy be withdrawn for solution to freeze.

  
• Osmotic pressure


  • The osmotic pressure of a solution is the difference in pressure between the solution and the pure liquid solvent when the two are in equilibrium across a semipermeable membrane.

  
• Types of solutions based on osmotic pressure


  • Isotonic
  • Hypotonic
  • Hypertonic

  
• Van't Hoff factor (i)

  The reason electrolytes have a greater effect on colligative properties than nonelectrolytes is that electrolytes dissociate into ions in solution, and so one unit of an electrolyte compound separates into two or more particles when it dissolves. Thus i for nonelectrolytes is 1 and for electrolytes is greater than 1.
• Ethylene glycol is a common automobile antifreeze that is water soluble and fairly nonvolatile.
• Calculating molar mass using colligative properties


  Determine the change in freezing point or boiling point

  2. Use the appropriate formula to calculate molality

  3. Use the molality and mass of solvent to calculate moles of solute

  4. Divide the mass of solute by the moles of solute to get the molar mass
• Colligative properties can be used to determine the molar mass of a solute.