Themes > Science > Chemistry > General Chemistry > Atomic Structure > Electronic Structures of Atoms > Avogadro's Number and the Mole > The Mole and the Molecular Mass

The mole is defined as the number of carbon 12 atoms in 12 grams of carbon 12. The number of atoms in 12 grams of carbon 12 atoms corresponds to Avogadro's Number, which is 6.02 * 1023. This may seem confusing, but just understand that:
  • 1 mole (of any substance) = 6.02*1023 atoms, molecules, or ions (depending on how that substance exists at that time.) (For example, 1 mole of Na+ ions has 6.02*1023 Na+ ions in it.)
  • 1 mole (of any substance) = the total of the elements atomic masses expresses in grams. (For example, 1 mole of Na weighs 23 grams.)

The mole plays many valuable roles in Chemistry because it allows Chemists to weigh substances and tell how many particals are in that substance. For instance, if there are 12 grams of Carbon 12 then there are also 6.02 * 1023 atoms of carbon 12, but this ratio also applies to all elements. For instance, if a student has 6.941 grams of lithium, then that same student has 6.02 * 1023 atoms of lithium. Because the mole is used so often in Chemistry, an abbreviation has been created as mol.

Molecular Mass (Molecular Weight)

If someone asked right now how much one mole of Helium weighs, the answer would be 4.003 grams, but if someone asked how much one mole of Al2S3 (Aluminum Sulfide) weighs, uh??? To do this there is a basic assumption that has to be put into place. This assumption is that in one mole of Al2S3, there are two moles of Al and three moles of S.

This assumption might seem strange, but look at it like this. If Bob has 3 pens, and he proceedes to pull off the caps, how many caps does he have? Of course he has 3! Well, how many pens (without the caps) does he have? Of course he has 3! The same principle applies to molecules and compounds. If Bob has 1 mole of Al2S3, and he decides to break it up into Al and S, then he would have 2 moles of Al and 3 moles of S.

So using this assumption, there are 2 moles of Al and 3 moles of S, so take 26.98 grams (which is the weight of 1 mole of Al) and multiply it by 2 (because there are two moles of Al in Al2S3). Then take 32.06 grams (which is the weight of 1 mole of S) and multiply it by 3 (because there are three moles of S in Al2S3). Then add the two amounts up.

Al+3 = 2 mol Al+3 = 2 (26.98) = 53.96 g

S-2 = 3 mol S-2 = 3 (32.06) = 96.18 g

Total - 150.14 g


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