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Non-volatile vs volatile solutes?

Discussion in 'MCAT Study Question Q&A' started by unsung, Jun 17, 2008.

  1. unsung

    10+ Year Member

    Mar 12, 2007
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    Resident [Any Field]
    How do we know if a solute is volatile or non-volatile? I guess salt NaCl is non-volatile because it lowers vapor pressure of the solution and raises Bpt. But what else?
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  3. Kaustikos

    Kaustikos Archerize It
    7+ Year Member

    Jan 18, 2008
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    Yeah, I think that's pretty much how I remembered it. If something is a volatile solute it competes for the vapor pressure owned by the solvent. Basically because it has its own vapor pressure as well. Thus, a volatile solute will decrease the vapor pressure for the solvent like you said.
    If you're asking how will we know on the MCAT, truth be told, I don't. I think that the MCAT always tells you this in the passage or question (ie: You add 0.1M of volatile pentene in aqeous alcohol...). I've never had them NOT specify the compound in question as volatile/non-volatile.
    Correct me if I'm wrong, but dont' volatile solutes also affect the boiling point of their solvent different than non-volatile?
  4. TJames

    2+ Year Member

    Feb 18, 2008
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    Post Doc
    A volatile solute is something that is itself volatile. When it is in solution it will have a non-zero vapor pressure.

    Also, adding a volatile solute to a solvent will not necessarily raise the boiling point. Adding a very low-boiling solvent such as diethyl ether to a very high-boiling solvent such as DMSO will lower the boiling point.

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