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Homework Help: Standard Entropy for Liquid at Melting Point

  1. May 21, 2014 #1
    1. The problem statement, all variables and given/known data
    The vapour pressures of a liquid have been measured and fit to the following equation:
    Log10 (mmHg) = -3571/T + 6.124
    The melting point has been determined to be 392.7 K.
    Calculate the standard entropy of the liquid at the melting point.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. May 21, 2014 #2


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    Gold Member

    Where's your attempt?
  4. May 22, 2014 #3
    My attempt was as follows:
    T1= 392.7 K P1 = 0.8045 mmHg (from the equation, giving the vapour pressure of the LIQUID at the melting point).
    T2= 298.15 K P2= 1.052E-3 mmHg

    From the Clausius Clapeyron:

    ln(0.8045/1.052E-3)= ΔH/R*(1/298.15 K - 1/392.7 K)

    ΔH/R = 8221.89
    ΔH = 68361 J/mol

    ΔH/T = ΔS = 68361/392.7 = 174.07 J/mol K

    However the answer is about twice this value and a Cp value given for the liquid is 250 J/mol K and the entropy of vaporisation is 117 J/mol K (2 additonal variables)
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