1. Physical Properties of Gases
Pressure of Gases

2. Pressure of Gases
The pressure of a gas corresponds to the force (in newtons, N) that it exerts of a surface (area, m2).
Two gases that are subjected to the same conditions will exert the same pressure.
Why?

3. Pressure of Gases
The force that is exerted on the surface is caused by the collisions of the gas particles; the greater the number of collisions, the greater the force that is exerted on the surface. A lighter gas will undergo more collisions because it can move quicker, however, due to the smaller particle size, it will exert a smaller force on the surface. Heavier gas particles will undergo fewer collisions due to their larger size, but the larger sized particles exert a greater for on the surface of the container.

4. Atmospheric Pressure
Gas particles are kept close to the Earth due to gravity. The particles exert a force on all objects and this force is the atmospheric pressure. Recall from earlier that particles are always moving and collisions occur. There are more particles closer to the Earth’s surface, so collisions occur more frequently.
Atmospheric pressure at sea level is kPa = 760 mm Hg = 760 torr = 1 atm.

5. Atmospheric Pressure
The U-tube manometer
A barometer is used to measure the atmospheric pressure and a manometer is used to measure the pressure in a container.
(c) Pgas= Patm - Php

6. The U-tube Manometer
Closed-end – The pressure exerted by the gas in the container is equal to the difference in height in mm Hg between the two columns of liquid in the U-tube.

7. The U-tube Manometer
Determining the pressure using a closed ended manometer:
Pgas = h
Where, Pgas is the pressure of the gas in mm Hg
h is the height of the column of mercury.
Example: The pressure of oxygen is measured using a closed-ended manometer. The level of mercury in the ample arm is 36.2 cm Hg. The level of mercury in the arm furthest from the sample is 98.1 cm Hg. What is the pressure of the oxygen in atm?
Pgas = h = 98.1 – 36.2 = 61.9 cm Hg = 619 mm Hg/760 mm Hg = atm

8. The U-tube Manometer

9. The U-tube Manometer