# Homework Help: Standard Entropy for Liquid at Melting Point

1. May 21, 2014

### DiffusConfuse

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. May 21, 2014

3. May 22, 2014

### DiffusConfuse

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)