Relative Mass & Avogadro's Hypothesis

Back when humans knew hardly anything about elements and their properties, scientists found the masses of several sustances: H, O, and CO2.  They found a very curious thing; that the same amounts of different gases had a constant mass ratio.
O:H always had a 16:1 ratio
CO2:O = 11:8

Relative mass: a ratio of the mass of an atom relative to 1/12th the mass of carbon (assigned a mass of 12 amu)

note: amu = atomic mass unit

First they used oxgen(16 amu) but later switched to carbon. 

This all works because of a great man called Avogadro.  He was an Italian scientist who discovered the mole(also called Avogadro's constant) and developed his hypothesis (Avogadro's hypothesis). 

Avogadro's Law- at the same temperature and pressure, equal volumes of different gases have the same number of particles. 


Check out this link for a more detailed explanation!
Avogadro's Law explanation-youtube clip