Measurement of Gas Pressures

As stated above, the pressure of a gas is the force it exerts per unit area. This force can push a
column of liquid up a tube: the higher the pressure, the larger the force for a given area, and
consequently the more liquid that is pushed up. This behavior is exploited in the measuring of
pressure. There are two different kinds of setups or devices: the barometer and the manometer.

The simplest form of barometer consists of a container of liquid open to the atmosphere, with an
inverted tube placed in it. Care is taken so that there is no air trapped within the inverted tube. The
pressure exerted by the air in the environment (i.e., the atmospheric pressure) pushes the liquid up
the tube. The pressure is reported as the height to which the liquid rises. A value of 760 mm Hg, for
example, which is a typical reading for atmospheric pressure, means that liquid mercury (Hg) rises
to a height of 760 mm.

If, instead of measuring just the atmospheric pressure, we wish to measure the pressure of a gas
generated in a reaction, a slightly different setup is needed, although it is one that uses the same
principle. In an open-tube manometer, the liquid (usually mercury) is placed in a U-tube with one
end exposed to the atmosphere and the other end connected to the closed vessel holding the gas
whose pressure we want to measure: