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Homework Help: Rault's Law, Finding Mass of an unknown.

  1. Sep 12, 2012 #1
    1. The problem statement, all variables and given/known data

    When 10.7 g of an unknown, non-volatile, non-electrolyte, X was dissolved in 100. g of water, the vapor pressure of the solvent decreased from 23.8 torr to 23.6 torr at 298 K. Calculate the molar mass of the solute, X.

    2. Relevant equations

    The answer is 227, but I know I'm doing wrong in the equation somewhere.

    3. The attempt at a solution

    100gH20 = 5.56 moles, Molar Fraction = .991

    .991 = 5.56 / 5.56 + x =

    5.51 + .991x = 5.56

    .05 / .991 = 05 <---subtracted

    10.7 / .05 = 214 g

    Would appreciate any help!
  2. jcsd
  3. Sep 12, 2012 #2


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    Staff: Mentor

    Don't round down intermediate values and you will be OK.
  4. Sep 12, 2012 #3
    Sigh I just got 227.7, I would have definitely rounded off but at least I got the answer now lol. Generally speaking, should I ever round off during the calculations? Some teachers do and some don't so I'm a little confused.
  5. Sep 12, 2012 #4


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    Staff: Mentor

    Never round intermediate values during calculations (you have just seen why). If you have to report them, report rounded, but in calculations either use all digits you got (if they are kept in calculator register), or - if you have to save some value to use it later, use at least 2 or 3 so called guard digits - that is, if you know that the number you calculated has 3 sig figs, write it down with 5 or 6 digits.
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