Avogadro's Principle

Equal volumes of  gas at the same temperature and pressure will contain the same number of gas particles. 

This means if a room is filled with helium gas or if it is filled with oxygen gas, propane gas, or a water vapor, or Chanel No. 5..., it will contain the same number of gas particles so long as the temperature and the pressure is the same.

People have measured this with respect to gases at standard temperature and pressure, finding that ....

22.4 liters of any gas at a temperature of 273K (0 C) and a pressure of 1 atm. (760 mm Hg) will contain one mole (6.02 x 10²³ gas particles (molecules or individual gas atoms)

22.4 liters/mole of any gas at STP

Avogadro's my name and I'm a gas!
Some said my theory just wouldn't last!
It's not hard.  It's safe to assume,
That the same number particles occupy the same volume!
I wrote the mole, the unit,
And it seems like on every test we always use it! 
I am not here to cause no trouble
I am just here to do the Super Mole shuffle! 

A slightly changed version of a verse from  "Shane Totten's (aka Captain Ecology) and Jeff Martis', Dave Wang's, and Matt Patterson's "Super Mole Shuffle" ca. 1986