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Stoichiometry II- Calculations Based on Chemical Equations
Moles of 0 2 = (0 458 ton moles O 2 ) ( 2 000
l *^L°JL V
g-moles O,\
\
ton-mole O 2 7\
Ib-mole O 2 /
= 4 16 x 10° moles O 2
The ideal gas equation will give the liters of O 2 at the specified conditions (77°F
= 25°C, and 0 9 atm = 0 9 x 760 torr/atm = 684 torr)
(4 16 x 10
5 moles) /62 4
torr llter ) (298 K )
_
nRT
__
V
mole
K
/
_
P
684 torr
= 1 13 x 10
7 liters of pure O 2
If A' = volume of air (21 0% O 2 by volume) that is required, then 0 210^ must
provide 1 13 x 10
7 liters of pure O 2
= 3 99 x I0
1 ft
3 of pure O 2
X =
3 9 p * '°° = 1 90 x I0
b ft
3 of air
PROBLEM:
A 0 2052 g mixture of copper and aluminum is analyzed for the percentage of
aluminum by adding an excess of H 2 SO 4 and collecting the evolved H 2 over water
(Copper will not react with H 2 SO 4 ) The volume of H 2 collected is 229 5 ml at a
barometric pressure of 732 torr The temperature is 29°C Calculate the percentage
of aluminum in the original sample The chemical reaction is
2A1 + 3H 2 SO 4 -» A1 2 SO 4 + 3H 2
or
2A1 + 6H
+ ^2Al
i+ + 3H 2
SOLUTION:
We follow the standard three simple steps ' to find, from the given amount of H 2 ,
how much Al must be present At the end, this amount of Al is stated in terms of
how much sample was used (that is, the % purity of the sample)
1 From the volume of H 2 given (produced), compute the number of moles
of H 2 that are produced, by using the ideal gas equation The pressure
must be corrected for the vapor pressure of water (30 torr at 29°C, from
Table 11-1)
PV
(732 torr - 30 torr)(0 2295 liter)
62 4
(3Q2 K)
\
mole K/
= 8 55 x 10
3 moles H 2