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An ideal gas is defined as one in which all collisions between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces.

Ideal gas law is a generalization containing both Boyle's law and Charles's law as special cases and states that:

In such a gas, all the internal energy is in the form of kinetic energy and any change in internal energy is accompanied by a change in temperature. An ideal gas can be characterized by three state variables:
  • absolute pressure (P),
  • volume (V),
  • and absolute temperature (T).
The relationship between them may be deduced from kinetic theory and is called the Ideal gas law.

PV = kT = nRT
where
  • n is the total number of moles,
  • NA = Avogadro's Number = 6.02217 · 1023 molecules/mole,
  • R = Universal gas constant = 8.314 J/K · mol ,
  • k = Boltzmann Constant = R/NA = 1.380622 · 10-23 J/K.
The ideal gas law can be viewed as arising from the kinetic pressure of gas molecules colliding with the walls of a container in accordance with Newton's laws. But there is also a statistical element in the determination of the average kinetic energy of those molecules. The temperature is taken to be proportional to this average kinetic energy; this invokes the idea of kinetic temperature.