Http://water.ky.gov/groundwater/Pages/GroundwaterAwareness.aspx
As Fire needs oxygen to burn, Carbon Dioxide Covers the fire like an blanket and cuts the contact of fire to the oxygen present in the air, So Fire stops burning.That is why Carbon Dioxide is Considere as the best Fire Extinguisher.
I believe its n= m/M
n - number of moles
m - mass of substance
M - molar mass of substance