Avagadro's Law portrays the relationship between moles and volume. Through his law we can see that when the moles increase the volume increases and what the moles decrease the volume decreases making this relationship proportional.
*Make sure to keep temperature and pressure constant
*Another important thing to remember is Avagadro's number: 6.02 X 10^23
Here is the equation you use to find the amount of moles in a gas:
http://thescienceclassroom.org/wp-content/uploads/2013/04/Avogadros-Law.png
Also here is a
link that might help!