Standard Entropy of a liquid at melting point with no S(298.15) given

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SUMMARY

The discussion centers on calculating the standard entropy of a liquid at its melting point, specifically at 392.7 K, using the Clausius-Clapeyron equation. The derived enthalpy change (ΔH) is calculated to be 68361 J/mol, leading to an entropy value of 229.29 J/mol K. However, the expected entropy value is 350.60 J/mol K, indicating a discrepancy that arises when attempting to incorporate the given ΔSvap of 117.20 J/mol K. The user seeks clarification on how to correctly apply the relationship L = TΔS to resolve the entropy values.

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  • Understanding of the Clausius-Clapeyron equation
  • Familiarity with thermodynamic concepts such as enthalpy (ΔH) and entropy (ΔS)
  • Knowledge of vapor pressure calculations and their significance
  • Basic proficiency in manipulating logarithmic equations
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  • Research the relationship between enthalpy and entropy in phase transitions
  • Learn how to accurately calculate vapor pressures from temperature data
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Chemistry students, thermodynamics enthusiasts, and anyone involved in physical chemistry or phase transition studies will benefit from this discussion.

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Homework Statement


Vapour pressures of a liquid have been measured and fit to the following equation:
Log10 P (mmHg) = -3571/T + 8.999
The melting point has been determined to be 392.7 K.
A Cp value given for the liquid is 250 J/mol K
and the ΔSvap is 117.20 J/mol K

Homework Equations


Clausius Clapeyron:

ln(P1/P2)= ΔH/R*(1/T2 - 1/T1)

The Attempt at a Solution



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

ΔH/R = 8221.89 K-1
ΔH = 68361 J/mol

ΔH/T = ΔS = 68361/298.15 = 229.29 J/mol K

However the answer is 350.60 J/mol K.

I tried adding the ΔSvap to the derived value (229.29+117.2) , however that only gives me 346.5 J/mol K
 
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Maybe this relation might be helpful: ##L = T\Delta S = T(S_2 - S_1)##.
 
I would use L= TΔS = 68361 J/mol = 229.29 J/mol K x 392.7 K
= 392.7(S2-S1),
At this point I am unsure what to use for S2 to solve for S1 or vice versa.
 

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