Avogadro's law states that, "equal volumes of all gases, at the same temperature and pressure, have the same number of molecules"
For a given mass of an ideal gas, the volume and amount (moles) of the gas are directly proportional if the temperature and pressure are constant.
Using the assumption of little spheres (for interatomic space) the spacing between the molecules at about 2X10-10 m i.e.
The size of an atom is about an angstrom (10 -10 m) . In solids, which are tightly packed, atoms are spaced about a few angstroms (2 Å) apart.
Considering a gas in a parallelopiped, in a small time interval , a molecule with x-component of velocity will hit the wall
Collision by moleculeto the wall of conntainer assumed to be perfectly elastic.
By the principle of conservation of momentum, the momentum imparted to the wall in the collision = 2mvx
So the magnitude of the force on the wall from each collision is
distance travelled in time