
Interpretation:
The vapor pressure of methanol at 25∘C is to be determined.
Concept introduction:
The relationship between vapor pressure and temperature is given by the Clausius–Clapeyron equation, expressed as:
log(P1P2)=ΔHvapR(1T2−1T1)
Here, ΔHvap is the molar heat of vaporization, R is the gas constant, P1&P2 are the vapor pressures, and T1&T2 are the temperatures.
Conversion factor forkilo joule per mole to joule per mole:
1 kJ/mol = 103 J/mol

Answer to Problem 141AP
Solution: 127 mm Hg
Explanation of Solution
To convert temperature from degree Celsius to kelvin, the expression is as follows:
K = oC+273
Here, K is the temperature in kelvin, oC is the temperature in Celsius.
Convert the temperature intokelvin:
T1 =(65oC+273.15)=338.15 KT2 =(25oC+273.15)=298.15 K
The reaction is as follows:
CH3OH(l)→CH3OH(g)
The molar enthalpy of vaporization is as follows:
ΔHvap=ΔH∘f[CH3OH(g)]−ΔH∘f[CH3OH(l)]=−201.2 kJ/mol−(−238.7 kJ/mol)=37.5 kJ/mol
Convert the heat in joule per mole as follows:
1 kJ/mol = 103 J/mol37.5 kJ/mol=37.5×103 J/mol
The relation between pressure and temperature is as follows:
log(P1P2)=ΔHvapR(1T2−1T1)
Substitute the values of pressure, temperatures, and enthalpy of vaporization,
ln(P1760 mm Hg)=37.5×103 J/mol8.314 J/K.mol(1338.15 K −1298.15 K)=4510.5(298.15 K−338.15 K338.15 K ×298.15 K)=−1.79
Usingantilogarithm on both sides as follows:
ln(P1760 mm Hg)=−1.79P1760 mm Hg=exp(−1.79)=0.1669
Rearrange the above expression to discern the value of pressure.
P1=0.1669×760 mm Hg=127 mm Hg
The vapor pressure of methanol at 25∘C is 127 mm Hg.
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Chapter 11 Solutions
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