Page 66 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 49

P vap = 95.2/760 = 0.125 atm
C = 1,780 mg/L = 1.78 g/L
= (1.78 g/L) ÷ (78.1 g/g-mole) = 0.0228 mole/L = 0.0228 M
So, H = (0.125 atm) ÷ (0.0228 M) = 5.48 atm/M

Discussion:
1. The calculated value, 5.48, is essentially the same as the value in
Table 2.5, i.e., 5.55.
2. It should be noted that values of vapor pressure, solubility, and
Henry’s constant mentioned in a technical article might come
from different sources. So the Henry’s constant derived from the
ratio of vapor pressure and solubility might not match well with
the stated value of the Henry’s constant.

Example 2.30: Use Henry’s Law to Calculate the
Equilibrium Concentrations
The subsurface of a site is impacted by tetrachloroethylene (PCE). A recent
soil vapor survey indicates that the soil vapor contained 1,250 ppmV of PCE.
Estimate the PCE concentration in the soil moisture. Assume the subsurface
temperature is equal to 20°C.

Solution:
(a) From Table 2.5, for PCE
H = 25.9 atm/M and MW = 165.8
Also, 1,250 ppmV = 1,250 × 10 atm = 1.25 × 10 atm = P A
-3
-6
Use Equation (2.19):
P = H C A
A
A
= 1.25 × 10 atm = (25.9 atm/M) × (C )
−3
A
So, C = (1.25 × 10 ) ÷ 25.9
−3
A
= 4.82 × 10 M = (4.82 × 10 mole/L)(165.8 g/mole)
−5
−5
= 8 × 10 g/L = 8 mg/L = 8 ppm
−3
(b) We can also use the dimensionless Henry’s constant to solve this
problem.
H = H RT = 25.9 = H (0.082)(273 + 20)
*
*
H = 1.08 (dimensionless)
*
Use Equation (2.1) to convert ppmV to mg/m :
3
1,250 ppmV = (1,250)[(165.8/24.05)] mg/m 3
= 8,620 mg/m = 8.62 mg/L
3