Page 185 - Theory and Problems of BEGINNING CHEMISTRY

174 GASES [CHAP. 12

Closed end
Almost complete vacuum

Vertical
tube
h

Air
pressure

Mercury

Fig. 12-1. Simple barometer
Air pressure on the surface of the open dish is balanced by the extra pressure caused by the weight of the mercury
in the closed tube above the mercury level in the dish. The greater the air pressure, the higher the mercury stands
in the vertical tube.

Note that the dimension 1 atmosphere (1 atm) is not the same as atmospheric pressure. The atmospheric
pressure—the pressure of the atmosphere—varies widely from day to day and from place to place, whereas the
dimension 1 atm has a fixed value by definition.
The unit torr is defined as the pressure necessary to support mercury to a vertical height of exactly 1 mm.
Thus, 1 atm is by definition equal to 760 torr.
The SI unit of pressure is the pascal (Pa).
5
1.000 atm = 1.013 × 10 Pa = 101.3kPa

12.3. BOYLE’S LAW
Robert Boyle (1627–1691) studied the effect of changing the pressure of a gas on its volume at constant
temperature. He measured the volume of a given quantity of gas at a given pressure, changed its pressure, and
measured the volume again. He obtained data similar to the data shown in Table 12-1. After repeating the process
many times with several different gases, he concluded that
At constant temperature, the volume of a given sample of a gas is inversely proportional to its pressure.

This statement is known as Boyle’s Law.

Table 12-1 Typical Set of Data
Illustrating Boyle’s Law

Pressure P (atm) Volume V (L)

4.0 2.0
2.0 4.0
1.0 8.0
0.50 16.0