The kinetic molecular theory is a simple model that attempts to explain the properties of an ideal gas. The postulates of the kinetic molecular theory:

Gases consist of particles, which have the following properties:

1. The particles are so small compared to the distances between them that the volume of the individual particles can be assumed to be zero.
2. The particles are in constant motion. The collisions of the particles with the walls of the container are the cause of the pressure exerted by the gas.
3. The particles are assumed to exert no forces on each other; they are assumed neither to attract nor to repel each other.
4. The average kinetic energy of a collection of gas particles is assumed to be directly proportional to the Kelvin temperature of the gas.

As expected, real gases do not conform to these assumptions, but they are accurate in explaining ideal gas behavior.

The average kinetic energy of a gas can be determined if given the temperature. The equation used is (KE)_avg = 1.5 RT, where R is equal to 8.3145 J/K mol (which is also equal to .08026 L atm/K mol, but different units). Temperature must also be expressed in Kelvin. Temperature can also be used to determine the root mean square velocity. Symbolized as u_rms, root mean square velocity is equal to (sq. root)(3RT)/M. In this equation R is equal to 8.3145 J/K mol, T must be Kelvins, and M is the mass of a mole of gas particles in kilograms. The root mean square velocity is in the units m/s. Although u_rms for oxygen gas at STP is about 500 m/s, the majority of the O₂ molecules are not actually going that fast. Instead, the actual distribution of the velocities is shown in the graph below.

Information provided by: http://library.thinkquest.org