Avogadro's Law states that in the same volume of any gas, at the same temperature and pressure, there is the same number of molecules or atoms.

Watch the following video which provides a visualization of Avogadro's Law.

At STP (see section 2), the volume of 1 mole of a gas contains 6.02 x 10²³ atoms or molecules of that gas.

At STP, the volume of 1 mole of gas is 22.4 Liters.

6.02 x 10²³ is called Avogadro's number, and represents the number of particles in 1 mole of any substance, not just gasses.

Source: Hot air balloon in flight quebec 2005, Mtruch, Wikimedia Commons

Source: Scientific Balloon, BotMultichillT, Wikimedia Commons

Source: Hydrogen Balloon, Wikimedia Commons, Saperaud

So, if you have 1 mole (4.00g) of Helium, you have 6.02 X 1023 atoms that have a volume of 22.4L at STP. If you have 1 mole of H2 gas (2.02g), you have 6.02 X 1023 molecules of H2 that have a volume of 22.4L at STP. If you have 1 mole of N2 gas (28.0g), you have 6.02 X 1023 molecules of N2 that have a volume of 22.4L at STP. If you have 1 mole of CO2 gas (44.0g), you have 6.02 X 1023 molecules of CO2 that have a volume of 22.4L at STP.

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