Chem Help! – Molality And Molarity Difference As To Freezing Point Boiling Point And Osmotic Pressure?

Explain why molality is used for boiling-point elevation and freezing-point depression calculations and molarity is used in osmotic pressure conditions.
Any help is greatly appreciated! thanks!

Tags:


This entry was posted on Monday, December 7th, 2009 at 2:41 am and is filed under Chemistry. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.

One Response to “Chem Help! – Molality And Molarity Difference As To Freezing Point Boiling Point And Osmotic Pressure?”

  1. Molality is used for colligative properties because the colligative properties depend only on the number of particles in solution . The number of particles is measure in moles. Molality does not depend on the temperature as does molarity. You see, if the temperature changes, then the volume changes slightly and the molarity can change. Also, the final volume of the solution will depend on characteristics of the solute as it goes into solution. The solute takes up some space and changes the volume, whereas if the mass of the solvent is used the molality will stay constant.
    Osmotic pressure uses an equation that looks remarkably like the ideal gas equation
    Ideal gas equation: PV = nRT
    solve for P
    P = nRT/V
    notice that n/V (moles per liter) = M
    P = MRT
    Osmotic pressure:
    P = iMRT where M is in moles / L
    Since R is in Latm/mol K and M is molarity.