SCH 3U1
AVOGADRO’S HYPOTHESIS
In 1803, the time when the idea of matter being a “particle” was being established, work was being completed on its nature and reactivity.
Englishman John Dalton suggested atoms of different elements have different masses. What were these masses?
Dalton worked on reacting elements and decomposing compounds. For example, when water was decomposed (to oxygen and hydrogen gas) he found the ratio of these masses 8:1. Dalton did not know how many atoms of each there were so he assumed there were one each. He thought the formula for water was HO.
At the same time Frenchman Joseph Louis Gay-Lussac measured the volumes that the reactants and products made. For the reaction with water he found that 2 L of hydrogen reacted with 1 L of oxygen to form 2 L of water. His findings can be summarized by Gay-Lussac’s Law of Combining Volumes: when gases react, they do so in simple, whole-number ratios by volume.
In 1811, Italian Amedeo Avogadro combined these two ideas.
He suggested that the gas particles themselves were not atoms but diatomic molecules. So, 2 H
2
+ O
2
 2 H
2
O. This statement is correct when considering volumes and numbers of particles.
This lead to Avogadro’s Hypothesis: equal volumes of gases at the same temperature and pressure contain an equal number of molecules.
For example, 1.0 L of H
2
at STP has the same number of molecules as 1.0 L of N
2
at STP. The mass of nitrogen is 14 times greater and is, therefore, 14 times more dense.