Using Avogadro's hypothesis to explain why hydrogen is less dense than helium
Amedeo Avogadro’s hypothesis states that equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties. According to our mole chart, there is one mole per 22.4 liters. However the amount of grams per mole is different varying on the gas. Helium has a molar mass of 4.0026 while hydrogen’s molar mass is 2.0158. At constant temperature and pressure (STP) the two gases have different weights but constant volume. Because density equals mass divided by volume, hydrogen is calculated to be less dense than helium.
Sources:
✿ http://wwwchem.uwimona.edu.jm/courses/pH/avono.html
✿ http://iweb.tntech.edu/chem281-tf/avogadro.htm
✿ http://chemwiki.ucdavis.edu/@api/deki/files/3010/=Avogadro.jpg
✿ http://creationwiki.org/pool/images/thumb/a/ab/-Amadeo_Avogadro.png/250px--Amadeo_Avogadro.png
✿ http://www.public.asu.edu/~jpbirk/CHM-113_BLB/Chpt10/img014.jpg