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:
The amount of the gas (in moles) The temperature of the gas (in K)
The volume of the gas (in L) 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
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/cm (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:
P gas= Pbar. P gas= P bar. ΔP, Pgas= P bar. ΔP,