Unformatted text preview: Also recall that the mathematical deﬁnition of activity ai of some species i is
implicitly stated as
ai
lim
=1
(46.9)
ζ →ζ ª g (ζ )
where g(ζ ) is any reference function (e.g., pressure, mole fraction, concentration
etc.), and ζ ª is the value of ζ at the reference state.
This implicit deﬁnition is awkward so for convenience one deﬁnes the activity
coeﬃcient as the argument of the above limit,
γi ≡ ai
g(ζ ) (46.10) which we can rearrange as
ai = γ i g (ζ ). (46.11) The deﬁnition of activity implies that γ i = 1 at g (ζ ª ) (the reference state)
That is γ i → 1 as the real system approaches the reference state.
Connecting with the chemical potential we saw last semester that the deviation
of the chemical potential at the state of interest versus at the reference state is
determined by the activity at the current state (the activity at the reference state
is unity by deﬁnition).
μi − μª = RT ln ai .
(46.12)
i 46.3.2. Raoult’s Law
In discussing both Raoult’s law and Henry’s law, we are describing the behavior
of a liquid solution by measuring the vapor (partial) pressures of the components 312 ...
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 Spring '09
 MAKIN
 Thermodynamics, Mole, reference state, RT ln ai

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