Non-Ideal Gases: How do you relate P, V, T?
Non-ideal if:
Step 1: Is the gas ideal or non-ideal? (Page 192)
RT
 20L / m ol (320 ft 3 / lb  m ol)
P
RT
 5L / m ol (80 ft 3 / lb  m ol) for diam tom icgas
P
If ideal, use PV = nRT
Step 2: Non-ideal? Choose an approach
(i) Equation of State
(ii) Compressibility Factor
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Compressibility factor “Z”
PV = znRT (z is “fudge factor”)
Calculate (2 of 3) Pr, Tr, Vr
Plots on p. 208-210
Find where Pr, Vr, or Tr intersect
and read Z
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Equations of State give analytic solutions
There are several EOS to choose from
SRK explained on p. 203
“plug and chug” to solve Eqn 5.3-7 below
RT
a
P 
  
V b V V b


PV = znRT
Notes:
1. Non-ideal gases use “reduced” values for P, T, V so that “universal” equations and
plots can be used. Otherwise, every gas (e.g., nitrogen, propane, etc) would have
different
plots and equations relating P, T, V.

V
2.
is the molar volume = volume divided by the moles (vol/mol)
3. Kay’s Rule: If you have more than one component, the “pseudocritical” T and P are
“weighted” by their mole fractions (pg. 211)
Pc  yA PcA  yB PcB  ....