Barrons SAT Subject Test Chemistry, 13th Edition

Unit

Abbreviation

Unit Equivalent to 1 atm

Atmosphere

atm

1 atm

Millimeters of Hg

mm Hg

760 mm Hg

Torr

torr

760 torr

Pascal

Pa

101,325 Pa

Kilopascal

kPa

101.325 kPa

A device similar to the barometer can be used to measure the pressure of a
gas in a confined container. This apparatus, called a manometer, is illustrated
below. A manometer is basically a U-tube containing mercury or some other
liquid. When both ends are open to the air, as in (1) in the diagram, the level of
the liquid will be the same on both sides since the same pressure is being exerted
on both ends of the tube. In (2) and (3), a vessel is connected to one end of the U-
tube. Now the height of the mercury column serves as a means of reading the
pressure inside the vessel if the atmospheric pressure is known. When the
pressure inside the vessel is the same as the atmospheric pressure outside, the
levels of liquid are the same. When the pressure inside is greater than outside, the
column of liquid will be higher on the side that is exposed to the air, as in (2).
Conversely, when the pressure inside the vessel is less than the outside
atmospheric pressure, the additional pressure will force the liquid to a higher
level on the side near the vessel, as in (3).

TIP

Know how to calculate the pressure in a closed vessel like in the manometer shown.