Pressure of Gases

Pressure is defined as force that is applied in a given area,  P = F/A.  Gases exert pressure on the surface in which they are in contact because the molecules of gases are always moving.

For the unit of pressure, force has the SI unit of  newton (N), where

                             1 N = 1 kg.m/s²

and the area's unit is m² and therefore, the SI unit of pressure is Pa which is equivalent to

                            1 Pa = 1 N/m²

What is atmospheric pressure?

Atmospheric pressure is the pressure exerted by earth's atmosphere.  The value of atmospheric pressure  depends on location, temperature and weather condition.  The atmosphere is much denser  near the surface of earth  than at high altitudes,  The denser the air is, the greater the pressure exerted.

Barometer is an instrument  used for measuring atmospheric pressure.  It was invented in 1643 by an Italian scientist named Evangelista Torricelli (1608-1647), who had been a student of the famous astronomer Galileo.  Below is the Torricelli's Barometer:

Standard atmospheric pressure, corresponds to the typical pressure  at sea level, is the pressure sufficient  to support a column of mercury 760 mm high at 0^oC at sea level.  In SI unit this pressure is equal to 101,325 Pa.

Standard atmospheric pressure defines some common non-SI units used to express gas pressure, such as atmosphere (atm), millimeter of mercury (mm Hg), and torr.

1 atm  =  760 mm Hg = 760 torr = 101,325 Pa = 101.325 kPa = 1.01325 bar = 14.69 psi (pounds per square inch,

Manometer is a device used to measure the pressure of gases other than the atmosphere.  The principle of operation is similar to that of barometer.  There are two types of manometer, the closed-tube and the open-tube manometer.

-Open-tube manometer is suited for measuring pressures equal to or greater than atmospheric pressure.

-Closed-tube manometer is used to measure pressures below atmospheric pressure.

Below are the models of the two types of manometer:

a.  Closed-tube manometer
b.  Open-tube manometer

Pressure Conversion

Sample Problem:

1.  Convert a.  0.075 torr to kPa

                   b.  6.6 x 10^-2 torr to atm

                   c.  749 mm Hg

Solution:

a.

b.

c.