Avogadro's Hypothesis: Equal volumes of different gases at the same temperature (T) and pressure (P) have the same number of particles (same number of moles).
Dalton's Law of Partial Pressures: The pressure of a gas is independent of the presence of other gases.
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We have been looking at the various properties of gases, now we want to look at a theory to explain those behaviors. A simple model is the kinetic-molecular theory. There are four basic postulates:
1) A gas is composed of a large number of tiny particles (molecules, or atoms for the inert gases). These particles are so small that the sum of the particles volumes is negligible compared to the volume of their container - most of the container volume is empty space.
Gases are easy to compress - expected if there is lots of empty space between them.
2) The particles of a gas are in rapid, linear motion. They make frequent collisions with each other and the walls of any vessel containing them. All collisions between gas particles and between gas particles and container walls are elastic.
This explains why gases rapidly fill their containers. We also note that they don't condense out as a liquid or solid if they are left in an insulated container (they don't lose energy as they collide with walls.) Brownian motion is also a consequence of their rapid movement.
3) Except when they are colliding, the particles are completely independent of each other. That is, there are no forces of attraction or of repulsion between them.
Three is a bit more subtle, and we won't worry about it.
4) The particle in a gas have a wide range of velocities: some may be nearly still, while others move at great speed. Thus there is a wide range of kinetic energies in any gas.
From this postulate we expect a distribution of velocities, as seen in the diagram below.
© R A Paselk
Last modified 18 November 2008