The Foundations of Chemistry

20,000 feet as it is at sea level. Mountain climbers and pilots use portable barometers to

determine their altitudes (Figure 12-2). At sea level, at a latitude of 45°, the average atmos-

pheric pressure supports a column of mercury 760 mm high in a simple mercury barometer

when the mercury is at 0°C. This average sea-level pressure of 760 mm Hg is called one

atmosphere of pressure.

1 atmosphere (atm)760 mm Hg at 0°C760 torr

The SI unit of pressure is the pascal(Pa), defined as the pressure exerted by a force

of one newton acting on an area of one square meter. By definition, one newton(N) is

the force required to give a mass of one kilogram an acceleration of one meter per second

per second. Symbolically we represent one newton as

1 N so 1 Pa

One atmosphere of pressure1.01325 105 Pa, or 101.325 kPa.

1 kg



ms^2

1 N



m^2

1 kgm



s^2

Figure 12-2 Some commercial
pressure-measuring devices. (a) A
commercial mercury barometer.
(b) Portable barometers. This type
is called an aneroid(“not wet”)
barometer. Some of the air has been
removed from the airtight box,
which is made of thin, flexible
metal. When the pressure of the
atmosphere changes, the remaining
air in the box expands or contracts
(Boyle’s Law), moving the flexible
box surface and an attached pointer
along a scale. (c) A tire gauge. This
kind of gauge registers “relative”
pressure, that is, the difference
between internal pressure and the
external atmospheric pressure. For
instance, when the gauge reads 30
psi (pounds per square inch), the
total gas pressure in the tire is
30 psi1 atm, or about 45 psi. In
engineering terminology, this is
termed “psig” (ggauge).

(b)

(a) (c)

Acceleration is the change in velocity
(m/s) per unit time (s), m/s^2.