PROBLEM 14.1
KNOWN:
Mixture of O
2
and N
2
with partial pressures in the ratio 0.21 to 0.79.
FIND:
Mass fraction of each species in the mixture.
SCHEMATIC:
2
O
N2
p
0.21
p
0.79
=
2
O
32 kg/kmol
=
M
2
N
28 kg/kmol
=
M
ASSUMPTIONS:
(1) Perfect gas behavior.
ANALYSIS:
From the definition of the mass fraction,
i
i
i
i
m
r
r
r
r
=
=
Σ
Hence, with
(
29
i
i
i i
i
i
i
p
p
p
.
R T
/
T
T
r
=
=
=
ℜ
ℜ
M
M
Hence
i i
i
i i
p /
T
m
p /
T
ℜ
=
Σ
ℜ
M
M
or, cancelling terms and dividing numerator and denominator by the total pressure p,
i
i
i
i
i
x
m
.
x
=
Σ
M
M
With the mole fractions as
2
2
O
O
0.21
x
p
/ p
0.21
0.21
0.79
=
=
=
+
2
2
N
N
x
p
/ p
0.79,
=
=
find the mass fractions as
2
O
32
0.21
m
0.233
32
0.21
28
0.79
×
=
=
×
+
×
<
2
2
N
O
m
1
m
0.767.
=

=
<
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