The Mole

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Picture or Amedeo Avogadro (1776 – 1856)

Which is heavier – a pound of feathers or a pound of bricks? This is a classic question posed to students in classrooms. Many of you can probably remember chuckling at the person next to you who said that the pound of bricks weighed more than the pound of feathers. Of course, they both way the exact same amount – a pound!

What is different about the pound of bricks and the pound of feathers is the amount of bricks or feathers we would have. The average brick weighs between 3 and 4 pounds so you may not even have a complete brick in your pound of bricks. While the average goose feather weighs around 0.00125 pounds so you’d be looking at around 800 feathers! So what is the difference between the brick and the goose feathers? Well the brick is a much more dense material than the goose feathers. In other words, the brick packs a lot more material into a much smaller space.

So how does this relate to Chemistry? Well lets say I had 12 grams of carbon atoms and I also had 12 grams of uranium atoms. Would I have the same number of atoms? I would actually have a much greater amount of carbon atoms than I would have uranium atoms because each uranium atom has a mass of about 20 times more than each carbon atom! This is just like our brick and feather problem.


Not this mole!

Since every element on the periodic table has an atom of a different mass, we needed a common way to group these atoms in order to understand how they interact with each other. There were a number of scientists who contributed to this new method of grouping and one of the largest contributor was Amedeo Avogadro. Avogadro was an Italian scientist who was famous for his “Avogadro Hypothesis” which states that two different gases of the same volume that are under the same conditions of temperature and pressure will contain the exact same amount of molecules. It was found that 22.4 L of any (ideal) gas contains 6.02 x 1023 molecules. This is a huge number and it gives an idea of how small atoms really are!

This number, 6.02 x 1023 is now called Avogadro’s Number or The Mole. It works in the same way that a dozen works. If I have one dozen eggs I have 12 eggs. If I have two dozen eggs I have 24 eggs. Likewise, if I have one mole of eggs I have 6.02 x 1023 eggs. If have a two moles of eggs I have 6.02 x 1023 eggs.

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