The kinetic theory of gases is a model that explains gas behaviour – pressure, temperature and volume – using a model of the gas as hard little particles of no volume which exert no forces on each other. For many everyday situations these are very good approximations. Classical Newtonian mechanics is used to derive the equation of state which describes the relationship between Pressure 
Volume
and Temperature
for an 'ideal gas'.
Suppose we have a cube full of N identical gas molecules of mass m inside a cube of side l. Imagine a gas molecule moving backwards and forwards between the walls 1 and 2. The collisions of the molecule with the walls of the box are perfectly elastic so no kinetic energy is lost, and the molecule always has speed v. We assume the wall to be fixed – this is not true, but they are so much heavier than the gas molecules that it is a very good approximation.
Each time the gas molecule collides with the wall (1) it's momentum changes by
Between each collision with wall 1 the molecule travels a distance
so takes
seconds to travel from 1 to 2 and back, hence the molecule collides with wall 1
times per second.
This means that the rate of change of momentum of the molecule is because of it's collisions with wall 1 is
and we can equate this to the impulse exerted by the wall on the molecule and hence using Newton's Third Law, to the impulse exerted by the molecule on the wall.
(1) since
We add up terms such as (1) obtaining
where N is the total number of molecules in the box, but now we have to take into account that not all the gas molecules are bouncing between walls 1 and 2: on average, only one third are, so we can introduce a factor of
to take account of this then
becomes
using
The mean kinetic energy of gas molecules is given by
number of mols*number of molecules in each mol
where
is Avagadro's number.
Finally
the Universal Gas constant:
This is the usually stated form of the ideal gas equation.