The Foundations of Chemistry

of the height of the mercury column, that is, the vertical distance between the surface of
the mercury in the open dish and that inside the closed tube. The pressure exerted by the
atmosphere is equal to the pressure exerted by the column of mercury.
Mercury barometers are simple and well known, so gas pressures are frequently
expressed in terms of millimeters of mercury (mm Hg, or just mm). In recent years the
unit torrhas been used to indicate pressure; it is defined as 1 torr1 mm Hg.
A mercury manometerconsists of a glass U-tube partially filled with mercury. One
arm is open to the atmosphere, and the other is connected to a container of gas (see Figure
12-1b,c).
Atmospheric pressure varies with atmospheric conditions and distance above sea level.
The atmospheric pressure decreases with increasing elevation because there is a decreasing
mass of air above it. Approximately one half of the matter in the atmosphere is less than
20,000 feet above sea level. Thus, atmospheric pressure is only about one half as great at

The unit torrwas named for

Evangelista Torricelli (1608–1647),

who invented the mercury barometer.

12-3 Pressure 437

Figure 12-1 Some laboratory devices for measuring pressure. (a) Schematic diagram of a
closed-end barometer. At the level of the lower mercury surface, the pressure both inside
and outside the tube must be equal to that of the atmosphere. There is no air inside
the tube, so the pressure is exerted only by the mercury column hmm high. Hence, the
atmospheric pressure must equal the pressure exerted by hmm Hg, or htorr. (b) The two-
arm mercury barometer is called a manometer. In this sample, the pressure of the gas
inside the flask is greater thanthe external atmospheric pressure. At the level of the lower
mercury surface, the total pressure on the mercury in the left arm must equal the total
pressure on the mercury in the right arm. The pressure exerted by the gas is equal to the
external pressure plusthe pressure exerted by the mercury column of height hmm, or
Pgas(in torr)Patm(in torr)htorr. (c) When the gas pressure measured by the
manometer is less thanthe external atmospheric pressure, the pressure exerted by the
atmosphere is equal to the gas pressure plusthe pressure exerted by the mercury column, or
PatmPgash. We can rearrange this to write Pgas(in torr)Patm(in torr)
htorr.

h (mm) h h

Atmospheric

pressure, Patm

Pressure

unknown

The pressure of the gas

in the flask is less than

atmospheric pressure.

(c)

Pgas (in torr) = Patm (in torr) – h torr

Atmospheric

pressure, Patm

Pressure

unknown

The pressure of the gas

in the flask is greater than

atmospheric pressure.

(b)

Pgas (in torr) = Patm (in torr) + h torr

Atmospheric

pressure

Mercury surface

(a)

Vacuum

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