Page 65 - Materials Chemistry, Second Edition

48 Practical Design Calculations for Groundwater and Soil Remediation

Example 2.28: Unit Conversions for Henry’s Constant
As shown in Table 2.5, the Henry’s constant for benzene in water at 25°C is
5.55 atm/M. Convert this value to dimensionless units and also to units of
atm.

Solution:
From Table 2.4
H = H RT = 5.55 = H (0.082)(273 + 25)
*
*
H = 0.227 (dimensionless)
*
Also, from Table 2.4
H = (H RT)[1,000γ/W]
*
= [(0.227)(0.082)(273 + 25)][(1,000)(1)/(18)] = 308.3 atm

Discussion:
1. As mentioned previously, use of the dimensionless Henry’s con-
stant is becoming more popular. Benzene is a VOC of concern and
is shown in most, if not all, databases of Henry’s constant values.
It may not be a bad idea to memorize that benzene has a dimen-
sionless Henry’s constant of 0.23 under ambient conditions.
2. To convert the Henry’s constant of another COC in the database,
just multiply the ratio of the Henry’s constants (in any units) of
that COC and of benzene by 0.23. For example, to find the dimen-
sionless Henry’s constant of methylene chloride, first read the
Henry’s constant for methylene chloride, 2.03 atm/M, and for
benzene, 5.55 atm/M, from Table 2.5. Then find the ratio of these
two and multiply it by 0.23, as [(2.03)/(5.55)] × (0.23) = 0.084.

Example 2.29: Estimate Henry’s Constant from
Solubility and Vapor Pressure

As shown in Table 2.5, the vapor pressure of benzene is 95.2 mm-Hg, and its
solubility in water is 1,780 mg/L at 25°C. Estimate the Henry’s constant of
benzene from the given information.

Solution:
From Equation 2.21, we know that Henry’s constant is the ratio of vapor
pressure and solubility, so

H = (95.2 mm-Hg) ÷ (1,780 mg/L) = 0.0535 mm-Hg/(mg/L)

To compare with the value given in Table 2.5, we need to do some con-
versions of the units: