Chemistry Reference
In-Depth Information
Since we have the number of moles of
oxygen,
we must use the partial pressure
of oxygen, which is the barometric pressure minus the water vapor pressure
(from Table 12.3):
P
O
2
762 torr
21.1 torr
741 torr
(0.0900 mol) (0.0821 L
#
atm/mol
#
K) (296 K)
(741 torr) (1 atm/760 torr)
nRT
P
V
2.24 L
Snapshot Review
❒
A
mixture
of gases behaves physically much like a gaseous sample of
a pure substance.
ChemSkill Builder 7.4
❒
The volume of each gas in a mixture is the same; the temperature of
each gas in a mixture is the same; the ratio(s) of the partial pressures
is (are) the same as the ratio(s) of numbers of moles.
❒
The ideal gas law can be used for the entire mixture or any single
component. If the entire mixture is used, the total pressure and the
total number of moles are used.
A. Calculate the mole ratio of gases A and B in a mixture if A has a partial
pressure of 0.500 atm and B has a partial pressure of 0.750 atm.
12.7
Molar Masses and Molecular Formulas
The ideal gas law may be used to determine the molar mass of a gaseous sub-
stance. To determine a molar mass, we need the mass of a given sample and
also the number of moles in that sample. The ideal gas law can be used to deter-
mine the number of moles.
EXAMPLE 12.21
Calculate the volume that 48.3 g of sulfur dioxide gas occupies at standard tem-
perature and pressure.
Solution
The molar mass of
SO
2
is used to calculate the number of moles of
SO
2
present:
a
1 mol SO
2
64.1 g SO
2
48.3 g SO
2
b
0.753
5
mol SO
2
The ideal gas law then enables us to calculate the volume under the stated
conditions:
(0.753
5
mol) (0.0821 L
#
atm/mol
#
K) (273 K)
1.00 atm
nRT
P
V
16.9 L
