Avogadro's law

Avogadro's dictates that the number of molecules in a specific volume of gas is dependent on it's pressure and temperature.

Simply,

n in mols is directly proportional to the gas' volume in liters n is proportional to it's pressure in kilopascals n is inversely proportional to it's temperature in Kelvin

Multiply all that by a constant to make an equality instead of a proportion and you get:

 P(kPa) V(L)   

n = ----------------

  R T(K)

Where R is equal to 8.31 (kPa * L)/(K * mol).

One mol occupies approximately 22.4 Liters at TPN.

TPN is equal to 101.3 kPa and 273 K

-Kaos