Untitled - Kelly Walsh High School

80 CHEMISTRY FOR THE UTTERLY CONFUSED
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Our goal in this chapter is to assist you in learning the concepts of gases and gas
laws. Be sure that you know how to properly use your calculator and, if you need
to, refer to Chapter 3 on the mole concept. It’s especially true with gas law problems
that the only way to master them is to Practice, Practice, Practice.
5-1 Gas Laws (P, V, n, and T)
Before we start describing the gas law relationships, we will need to describe
the concept of pressure. When we use the word pressure with respect to gases,
we may be referring to the pressure of a gas inside a container or we might be
referring to atmospheric pressure, the pressure due to the weight of the atmosphere
above us. The pressure at sea level is 1 atmosphere (atm). Commonly, the
unit torr is used for pressure, where 1 torr 1 mm Hg (millimeters of mercury),
so that atmospheric pressure at sea level equals 760 torr. The SI unit of pressure
is the pascal (Pa), so that 1atm 760 mm Hg 760 torr 1.01325 10 5 Pa (or
101.325 kPa).
Dalton’s law says that, in a mixture of gases (A B C . . . ), the total pressure
is simply the sum of the partial pressures (the pressures associated with
each individual gas). Mathematically, Dalton’s law looks like this:
P Total P A P B P C . . .
If you know how many moles of each gas are in the mixture and the total pressure,
you can calculate the partial pressure of each gas by multiplying the total
pressure by the mole fraction of each gas:
P A (P Total) (X A)
The term X A refers to the mole fraction of gas A. The mole fraction of gas A is
equal to the moles gas A/total moles of gas in the mixture. The mole fraction is
a concentration unit that we will see again in Chapter 12.
The gas laws relate the physical properties of pressure (P), volume (V), temperature
(T), and amount (n) to each other. If we keep the amount (number of