Avogadro's LAw

Avogadro's Law tells us that for a gas at constant temperature and pressure, the volume is directly proportional to the number of moles of gas. For example, if the moles of a gas present is increased, the volume will increase in order for the pressure of the system to remain constant. Also, equal volumes of gasses at the same temperature and pressure have the same number of particles.

Helpful Links
http://www.chemistry.co.nz/avogadro.htm
http://www.britannica.com/science/Avogadros-law