Unit 5 - Counting Particles Too Small to See

How many gum balls are in the machine? Counting them would certainly be a chore. However, it would pale in comparison to counting atoms and molecules. How did scientists figure out how many particles are in a give sample of a substance if these particles are invisible?



From Avogadro's Hypothesis we are able to count molecules by weighing macroscopic samples.

For gases at the same temperature and pressure we can deduce the following:
1. From combining volumes we can determine the ratio in which molecules react.
2. From masses of these gases we can determine the relative mass of individual molecules.
From these results it is possible to determine the molar masses of the elements; using these masses and formulas of compounds, one can determine molar masses of compounds. The mole was defined in terms of a readily weighable “lump” of a substance. These tools allow one to relate “how much stuff” to “how many particles".


Dr. Michael M. Krop High School







Unit 5 Materials


m r . m o n t e r o . c o m





Amedeo Avogadro (1776-1856) born in Italy, proposed a hypothesis which explained Gay-Lussac's law of gases reacting in integer volume ratios. Avogadro proposed that equal volumes of any gases, at the same temperature and pressure, contain the same number of molecules. He also proposed that some gases are diatomic molecules. Avogadro's hypothesis was not accepted until years after his death.