The equation of state for an ideal gas :

`PV = nRT`

In this expression, known as the ideal gas law, R is a universal constant that is the same for all gases and T is the absolute temperature in kelvins.

Experiments on numerous gases show that as the pressure approaches zero, the quantity PV/nT approaches the same value R for all gases.

R is called the universal gas constant.

In SJ units, in which pressure is expressed in pascals and volume in cubic meters, the product PV has units of newton meters, or joules, and R has the value

`R = 8.315` J/mol K

If the pressure is expressed in atmospheres and the volume in liters then R has the value

`R = 0.08214` L atm/mol K

Using this value of R we find that the volume occupied by 1 mol of any gas at atmospheric pressure and at `0^oC` (273 K) is 22.4 L.

`text(Assumption of kinetic theory of gases :)`

a. All gases are made of molecule moving randomly in all directions.
b. The size of a molecule is much smaller than the average separation between the molecules.
c. The molecules exert no force on each other or on the walls of the container except during collision.
d. All collisions between two molecules or between a molecule and a wall are perfectly elastic. Also, the time spent during a collision is negligibly small.
e. The molecules obey Newton's laws of motion.
f. When a gas is left for sufficient time, it comes to a steady state. The density and the distribution of position, direction and time. This assumption may be justified if the number of molecules is very large.