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 1 
Physics 2213 HW #1 — Solutions
Fall 2008
18.6
[Balloons]
We have
pV
nRT
=
for an ideal gas and
tot
m
nM
=
for the mass of the gas. The temperature must be
converted to Kelvin:
22.0 C
295.15K.
T
=
° =
The average molar mass of air is
3
28.8 10 kg mol
M

=
×
.
For helium
3
4.00 10 kg mol
M

=
×
. Next, we need to check units for pressure and volume, and make
sure they are consistent with those of the gas constant
R
. Since we are using SI units in this course, we
need to convert the pressure value from atm to Pa, and the volume from L to m
3
:
5
1 atm
1.01 10 Pa
p
=
=
×
, and
4
3
0.9 L
9 10 m
V

=
= ×
.
(a)
5
4
3
3
3
tot
(1.01 10 Pa)(9 10 m )(28.8 10 kg/mol)
1.07 10 kg .
(8.31 J/mol K)(295.15 K)
pV
m
nM
M
RT



×
×
×
=
=
=
=
×
⋅
(b)
5
4
3
3
4
tot
(1.01 10 Pa)(9 10 m )(4 10 kg/mol)
1.48 10 kg .
(8.31 J/mol K)(295.15 K)
pV
m
nM
M
RT



×
×
×
=
=
=
=
×
⋅
18.7
[Jaguar XK8]
We are asked to compare two states. Use the ideal gas law to obtain
2
T
in terms of
1
T
and ratios of
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 Fall '07
 PERELSTEIN,M
 Magnetism, Mass, Heat

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