Properties of Gases: Gas Pressure
•Gases expand to fill their container and assume the shape of their containers
•They diffuse into one another and mix in all proportions.
Four properties determine the physical behaviour of a gas:
1. The amount of the gas (in
2. The volume of the gas (in L)
3. The temperature of the gas (in
4. Pressure of gas (in atm or KPa)
The Concept of Pressure:
Consider this. A balloon expands when inflated with air, but what maintains
the balloon’s shape? One good argument is that the molecules of the gas within the
balloon are in constant motion and thus colliding with each other as well as the
walls of the container, the balloon, keeping the balloon in shape.
However it is difficult to measure the total force exerted by a gas. So in
chemistry, we speak in terms of pressure. Pressure is the force per unit area. In
translation, it is the force divided by the area over which the force is distributed.
It is difficult to measure the pressure of gas directly. So it is done indirectly
by comparison with liquid pressure. Liquid pressure depends only on the height of
the liquid column and the density of the liquid. Thus the formula, g x h x d, can be
derived. Since g is a constant, liquid pressure is directly proportional to the liquid
density and the height of the liquid column.
Barometric Pressure: (read experiment briefly, not important) the height of
mercury in a barometer, a measure of barometric pressure, varies with
atmospheric conditions and altitude. The standard atmosphere (atm) is defined
as the pressure exerted by a mercury column of exactly 760 mm in height when the
density of mercury = 13.5951 g/cm3 (at 0 ºC). Therefore, it is justified to say 1 atm
= 760 mmHg.
Manometers: just remember how that thing looks like, and these three situations:
1. Pgas = Pbar. 2. Pgas = P bar. +
P, ( P>0)Δ Δ
3. Pgas = P bar. +
P, ( P<0)Δ Δ
Boyle’s Law: For a fixed amount of gas at a constant temperature, the gas volume
is inversely proportional to the gas pressure.
P1V1 = n = P2V2