Page 60 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 43

Solution:
From Table 2.2, the vapor pressure of benzene is 95 mm-Hg at 25°C.
95 mm-Hg = (95 mm-Hg) ÷ (760 mm-Hg/1 atm) = 0.125 atm

The partial pressure of benzene in the pore space is 0.125 atm (125,000
× 10 atm), which is equivalent to 125,000 ppmV.
−6
Discussion:
This 125,000-ppmV value is the vapor concentration in equilibrium
with the pure benzene solution. The equilibrium can occur in a con-
fined space or a stagnant phase. If the system is not totally confined,
the vapor tends to move away from the source and creates a con-
centration gradient (i.e., the vapor concentration decreases with the
distance from the liquid). However, in the vicinity of the solution,
the vapor concentration would be at or near this equilibrium value.

Example 2.25: Using the Clausius-Clapeyron Equation
to Estimate the Vapor Pressure
The enthalpy of vaporization of benzene is 33.83 kJ/mol [8] and the vapor
pressure of benzene at 25°C is 95 mm-Hg (from Table 2.2). Estimate the vapor
pressure of benzene at 20°C using the Clausius-Clapeyron equation.
Solution:
Heat of vaporization = 33.83 kJ/mol = 33,830 J/mol
R = 8.314 (J)/(g mol)(K) from Table 2.1
Using Equation (2.17), we obtain

95 33,830  1 1 
ln sat =−  − 
2 P 8.314  (273 + 25) (273 + 20) 

P of benzene at 20°C = 75 mm-Hg
sat
Discussion:
As expected, the vapor pressure of benzene at 20°C is smaller than that
at 25°C. The difference is approximately 20% (75 vs. 95 mm-Hg).

Example 2.26: Using the Antoine Equation to
Estimate the Vapor Pressure
The empirical constants of the Antoine equation for benzene are [9]

A = 15.9008
B = 2788.51
C = −52.36