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Molar Mass question

  1. Jan 18, 2005 #1
    I had a multiple choice question like this:

    Which one of these CANNOT determine the molar mass?

    a) osmotic pressure

    b)change in temperature of freezing point

    c)vapor pressure measurements

    d)specific heat

    e)density of solid

    I know its not a or b. However i cannot decide between c, d, or e....
    I had d as well because you cannot determine the molar mass for everything. According to petit-dulong law, it only applies to metal elements. And even still, you cannot really determine the molar mass directly(can only estimate), where as osmotic pressure you can find molar mass directly and precise if values are correct.

    please help thanks.
    Last edited: Jan 18, 2005
  2. jcsd
  3. Jan 19, 2005 #2


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    You can rule out (c) likewise. It's a colligative property. After all, the BP is the temperature at which VP = 1 atm.

    Anyway, like you said, it's a bit of a toss-up between (d) and (e). I'll let you decide.

    d) works approximately for metals using Dulong-Petit... pretty accurately (within ~3% accuracy) for most elemental metals (Hg is off by ~15%); less accurately for alloys. Doesn't work for very much else.

    e) works very well for a crystalline solid, if you know its crystal structure. But this requires aqdditional information (ie : the crystal structure). Barely works for amorphous materials, though there's a rough correlation among some classes of organic compounds.
  4. Jan 20, 2005 #3
    You can use a,b,c if you are sure that the molecule will not dissociate/associate.
    Infact they were used .Then people noticed that it gives stupid results with some molecules due to association/dissociation,they were used to find degree of dissociation.Look up vant Hoff factor and all that.
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