The average kinetic energy of the particles of a substance is determined by the temperature of the medium, using the equation for an ideal gas. If the temperature is unknown, then the average speed and mass of the particles are utilized to determine the average kinetic energy.
Use the Maxwell-Boltzmann velocity distribution equation to calculate the average speed of the particles in a substance. Then, combine that information with the ideal gas law to calculate the average kinetic energy of a group of particles. However, there are certain assumptions, as all the energy is confined to translational motion. Internal energy, resulting from vibrational and rotational motion, is also discounted.