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VAPOR Pressure

Discussion in 'DAT Discussions' started by osimsDDS, Jun 20, 2008.

  1. osimsDDS

    osimsDDS 5+ Year Member

    Jun 8, 2007
    The dreaded vapor pressure anomaly...

    It took me a while but i think i FINALLY figured out vapor pressure...So basically the higher the vapor pressure the lower the temperature the solution boils...the lower the vapor pressure the higher the temperature the solution boils...

    Raoults law explains that as you add a solute to a pure solution the vapor pressure lowers (known as vapor pressure lowering) and therefore the temperature that the solution boils at now is higher than that of the pure solution.

    This can also be explained in the following examples of compounds (not in solution):

    CH3CH2OH, high boiling point = low vapor pressure
    CH3COOH, carboxylic acid = high boiling point = low vapor pressure


    CH3CH2CH3, dispersion forces = low boiling point = high vapor pressure

    As we know from organic chemistry the longer the alkane chain the more dispersion forces the higher the bp so for example pentane would boil at a higher temp than propane..etc

    hope this helps cuz it sure helped me!!!!
    Last edited: Jun 20, 2008
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  3. Dencology

    Dencology 2+ Year Member

    Mar 7, 2008
    yes, i just want to add one more thing that when the atmospheric pressure equal the vapor pressure water boils which i know all of you are aware of this.
  4. Glycogen

    Glycogen 2+ Year Member

    Jan 24, 2008
    I would not bring the alcohol example to prove that solute added to pure solvent would decrease vapor pressure=increase B.P b/c the increase of B.P in alcohol compound is due to hydrogen bonding.
    For vapor pressure depression(lower) which is one of the properties of colligative properties I would bring something like NaCl an ionic compound which when dissociate in the water would do all the things related to colligative properties such as : lower vapor pressure
    osmotic pressure
    freezing point depression
    higher boiling point

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