Page 184 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 12

Gases

12.1. INTRODUCTION
Long before the science of chemistry was established, materials were described as existing in one of three
physical states. There are rigid, solid objects, having a definite volume and a fixed shape; there are nonrigid
liquids, having no fixed shape other than that of their containers but having definite volumes; and there are gases,
that have neither fixed shape nor fixed volume.
The techniques used for handling various materials depend on their physical states as well as their chemical
properties. While it is comparatively easy to handle liquids and solids, it is not as convenient to measure out a
quantity of a gas. Fortunately, except under rather extreme conditions, all gases have similar physical properties,
and the chemical identity of the substance does not influence those properties. For example, all gases expand
when they are heated in a nonrigid container and contract when they are cooled or subjected to increased pressure.
They readily diffuse through other gases. Any quantity of gas will occupy the entire volume of its container,
regardless of the size of the container.

12.2. PRESSURE OF GASES
Pressure is defined as force per unit area. All fluids (liquids and gases) exert pressure at all points within
them in all directions. For example, in an inflated balloon, the gas inside pushes against the interior walls of the
balloon with such force that the walls stretch. The pressure of the gas is merely the force exerted on the interior
surface of the balloon divided by the area. The pressure of a gas is equal to the pressure on the gas. For example,
if the atmosphere presses on a piston against a gas with a pressure of 14.7 pounds per square inch (abbreviated
2
2
lb/in. ), then the pressure of the gas must also be 14.7 lb/in. . A way of measuring the pressure of the atmosphere
is by means of a barometer (Fig. 12-1). The standard atmosphere (abbreviated atm) is defined is the pressure that
◦
will support a column of mercury to a vertical height of exactly 760 mm at a temperature of 0 C. It is convenient
to express the measured gas pressure in terms of the vertical height of a mercury column that the gas is capable
of supporting. Thus, if the gas supports a column of mercury to a height of only 152 mm, the gas is exerting a
pressure of 0.200 atm:
1 atm

152 mm = 0.200 atm
760 mm
EXAMPLE 12.1. What is the pressure in atmospheres of a gas that supports a column of mercury to a height of
882 mm?
1 atm

Ans. Pressure = 882 mm = 1.16 atm
760 mm
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