Environmental Engineering Reference
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Ideal dilute
solution
Henry's law
P i = K H x i
Ideal solution
Raoult's law
P i = P i * x ii
X i
FIGURE 3.1 Illustration of Henry's law and Raoult's law. For a component that is pure (the
solvent) and behaves ideally it follows Raoult's law, where the partial pressure is proportional
to the mole fraction. When it is the minor component (the solute) in a dilute solution, its partial
pressure is again proportional to its mole fraction, but has a different proportionality constant,
which is the Henry's law.
E XAMPLE 3.7 D IFFERENT U NITS FOR H ENRY'S L AW C ONSTANT
The Henry's law constant (molar concentration ratio, K aw ) for benzene is 0.225.
Calculate the value in other units ( K X , K H , and K aw ) .
Use Table 3.5. Note V w = 0.018 L/mol and V a = 22.4 L/mol. Hence, K H = ( 0.225 )
( 0.08205 98 / 0.018 ) = 306 atm ( = 3.1 × 10 7 Pa), K X = 22.4 306 /( 0.082205
298 ) = 280 , and K aw = 0.018 306 = 5.5 L atm/mol ( = 5.6 × 10 5 Pa dm 3 /mol).
E XAMPLE 3.8 A CTIVITY C OEFFICIENT AND R AOULT'S L AW
Consider a mixture of chloroform and propanone for which the partial pressure
of chloroform at a liquid mole fraction of 0.2 is 35Torr. If the pure compound vapor
pressure of chloroform is 293Torr, calculate the activity coefficient of chloroform in
the mixture.
P i
P i =
35
293 = 0.12.
a i =
Hence
a i
x i =
0.12
0.20 = 0.597.
γ i =
 
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