Avogadro's Number - Counting Atoms by the Dozen (or by the Mole)

1. When doing reactions chemists need to count atoms and molecules. The problem of actually counting individual atoms and molecules comes from the fact that they are so small. Instead of physically counting each one we can measure the mass of a large group of them. If we know the mass of just one atom or molecule then we can figure out how many we have.
2. Because we will always be dealing with groups of atoms and molecules it would be convenient to give a name to a certain number of them. Just as we can talk about groups of objects like a dozen (12) eggs or a gross (144) of pencils, we need a unit for a large number of atoms or molecules. This unit is the mole.
3. 1 mole = 6.02 x 10²³. This is also known as Avogadro's number, in honor of his contributions toward the development of atom counting experiments and theories.
4. The atomic mass from the periodic table tells you how many grams of that element is needed to have a mole of those atoms.
   1. One mole of carbon atoms has a mass of 12.01g. For conversions between moles and grams: 1 mol = 12.01g
   2. One mole of helium atoms has a mass of 4.003g. For conversions between moles and grams: 1 mol = 4.003g
   3. One water molecule is made from two hydrogen atoms and one oxygen atom, so one mole of water molecules has a mass equal to two moles of hydrogen atoms and one mole of oxygen atoms. For conversions between moles and grams: 1 mol = 18.02g
      

      The formula for water is H2O, so a mole of water means a mole of these -->		2 x 1.008g = 2.016g 1 x 16.00g = 16.00g
      			18.02g

5. Practice Conversions:
   

   What is the mass of 2.60 moles of Carbon (C)?	What is the mass of 5.62 moles of oxygen (O2)?
   How many moles of carbon dioxide (CO2) do you have if you have 22.00g of it?	How many moles of butane (C4H10) are in a typical light containing 25.0g of the liquid?