Lecture 20: Gases: wrap up
Combined Gas Law:
2-state problems
P
1
V
1
n
1
T
1
= R
P
2
V
2
n
2
T
2
=
If
n
remains constant (same system):
P
1
V
1
T
1
P
2
V
2
T
2
=
combined gas
law
Always use T
in Kelvins

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P
1
V
1
T
1
P
2
V
2
T
2
=
Combined Gas Law
A gas occupies 401 mL at
P
= 1.000 atm. What will
be its volume if
P
is decreased to 0.750 atm?
T
is
held constant.
T
1
= T
2
(cancel out)
P
1
V
1
P
2
V
2
=
1.000 atm (401 mL) = (0.750 atm)
V
2
V
2
=
401 mL (1.000 atm/ 0.750 atm) = 535 mL
Gas Density and Molar Masses
For a gas with mass,
m,
and molecular mass,
M:
PV = nRT =
RT
m
M
d
is proportional to
M
.
M
(g/mol)
d
( g / L)
He
4.003
0.179
air
28.8*
1.28
O
2
31.999
1.428
SF
6
146.06
6.516
STP densities; *Average mass.
m
V
PM
RT
d =
=