What is an example of a thermochemistry practice problem?

1 Answer
Chuck W.
Oct 17, 2014

The boiling point of methanol is 64.7 C, and its enthalpy of vaporization is 37 kJ/mol. Calculate the equilibrium vapor pressure of methanol at 32 C.

Hint:
Use the phase equilibrium expression
ΔS_(vap) = (ΔH_(vap))/(T_b)
with the data provided to compute K_(eq). Remember that the activity of the liquid is 1, so K_(eq)=P_(vap)

Answer: 0.2438 bar

Solution:
ΔS_(vap) = (ΔH_(vap))/(T_b) = 109.5 J/(mol-K)
ΔG_(vap)(T) = ΔH_(vap) - TΔS_(vap) = 3581. J/(mol) at T = 305.15 K

The equilibrium constant K_(eq) = P_(vap) = e^((-ΔG_(vap))/(RT)) = 0.2438 (bar)

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