Untitled - Kelly Walsh High School

92 CHEMISTRY FOR THE UTTERLY CONFUSED
Quick Tip
For some reason, many students rearrange the ideal gas equation incorrectly;
you should be very careful to check your rearrangement of this equation.
Another type of gas law problem involves stoichiometry. Gas stoichiometry
problems are just like all other stoichiometry problems—you must use moles.
In addition, one or more gas laws are necessary. Let’s look at a gas stoichiometry
problem. What volume, in liters of oxygen gas, collected over water, forms
when 12.2 g of KClO 3 decompose according to the following equation:
2 KClO 3(s) → 2 KCl(s) 3 O 2(g)
The temperature of the water (and the gas) is 20.C, and the total pressure is 755
mmHg. (The vapor pressure of water at 20C is given as 18 mmHg.) In this case,
we not only need to separate the numbers (and associated units) but also we
need the balanced chemical equation.
2 KClO3(s) → 2 KCl(s) 3 O2(g) 12.2 g 20.C 293 K T
Ptotal 755 mmHg
18 mmHg
? L V
P H2 O
The presence of only one temperature, pressure, and volume indicates that we
need to use the ideal gas law. In order to find the volume of oxygen we need the
pressure of oxygen, not the total pressure, nor the vapor pressure of water. To
find the oxygen pressure we need a second gas law, Dalton’s law. In this case,
Dalton’s law has the form P total P O2 P H2 O, giving P O2 P total P H2 O (755
18) mmHg 737 mmHg.
We can now rearrange the ideal gas law (PV nRT) to isolate volume. Once
rearranged, we can enter the values given:
V nRT
P
L atm
(n) a0.0821 b(293 K)
mol K
(737 mmHg)