Chemistry Reference
In-Depth Information
Gases exert pressure, because their particles bombard the surface of a
material they come in contact with. At this moment, billions of gas par-
ticles, which make up the air around you, are striking your body every sec-
ond. Under normal circumstances, your internal pressure keeps these
collisions from having any noticeable effect on you. Sometimes the differ-
ence between the external air pressure and your internal pressure is evi-
denced by a discomfort in your eardrums. When your ears “pop,” your body
attempts to compensate for the difference in pressure.
The atmospheric pressure is measured with an instrument called a ba-
rometer, which is also called a closed manometer. When you watch your local
weather report, you will often hear the meteorologist discuss the barometric
pressure, which is a measure of the force exerted by the atmospheric gases
per unit of area. The news often reports pressure in “inches of mercury,”
because a barometer, as are many thermometers, is filled with mercury.
The SI unit for pressure is the Pascal, which is derived from Newtons and
meters. Other units that are commonly used to measure pressure include
the atmosphere (atm), torr, and millimeters of mercury (mm of Hg). You
should make sure that you know how to convert between the various units
of pressure. The necessary conversion factors are shown here.
Conversion Factors for Units of Pressure
1.0 atm = 101.3 kPa = 760 torr = 760 mm of Hg
1.0
kPa = 7.5 mm of
Hg
If you want to convert inches of mercury to any of the other units, you
will want to remember that 1 inch is approximately 25.4 mm. An example
of such a conversion is shown in the following example:
Example 1
The local news reports the atmospheric pressure as 30.03 inches of
mercury. How many kilopascals does this represent?
30.03 inches of Hg =
25.4 mm
inch
1 kPa
7.50 mm of Hg
×
= 101.7 kPa
This is also a good place to review the concept of “standard temperature
and pressure.” Because the volume of a gas is so susceptible to changes in
temperature and pressure, you must always make note of the temperature
and pressure conditions that the volume of the gas refers to. As you should
recall from Chapter 7, if you collected a 2.50 L sample of a gas in your labo-
ratory on a day when the temperature is low and the atmospheric pressure is
