Avogadro’s Number, the Mole and Molecular Weight

When chemists want to talk about chemical reactions on a realistic scale they do not count up
each individual atom and molecule.  Although it is possible to understand every chemical
reaction in terms of the component atoms and molecules it is not useful when doing
calculations.  Because there are trillions and trillions of water molecules in a single drop of
water, you would never want to add up the number of reactions that it would take to produce a
drop of water from hydrogen and oxygen molecules.  To solve this problem chemists do
calculations in terms of large groups of molecules, not individual ones.

Chemists use a very large number called a mole to deal with reactions between visible,
physically tangible amounts of substances.  One mole equals 6.02 x 1023.  This is also known
as Avogadro’s number and it is equal to a 6 followed by 23 zeros.  This number was chosen
because it happens to have some interesting properties.  For instance, one mole of hydrogen
atoms (6.02 x 1023 H atoms) weighs exactly 1 gram.

If you look to the periodic table it is apparent that a Helium (He) atom weighs four times as
much as a hydrogen (H) atom.  Because we know that a mole of H atoms weighs 1 gram it is
easy to see that a mole of He atoms would weigh 4 grams.  It is actually quite convenient, and
also 100% intentional, that the mass of one mole of any element is equal to the atomic mass of
that element in grams.

Using moles also allows us to easily make calculations concerning compounds- mixtures of
different molecules and elements.  In fact, if you know the molar mass of an element and you
also know how many elements make up a specific molecule in a compound, then it is possible
to calculate the molar mass of a compound by adding up the atomic weights.

Let say that we wanted to calculate the mass of a mole of water.  We know that water is one
part oxygen and two parts hydrogen so one mole of water will contain one mole of oxygen
atoms and two moles of hydrogen atoms.  A mole of hydrogen weights 1 gram, and the
periodic table tells us that a mole of oxygen weighs 16 grams.  So, we know that a single mole
of water will be equal to 18 grams.

(2 moles H  x  1 gram per mole) + (1 mole O  x  16 grams per mole) = 18 g

The mole is also useful in calculating mass in chemical reactions.  The coefficients in a
chemical equation correspond to the molar proportions of both the reactants and products.  

2H2 + O2  2H2O

In terms of just atoms and molecules the equation above tells us that 2 molecules of H (both
with 2 H atoms) react with 1 molecule of O (with 2 O atoms) to form 2 H2O molecules.  In terms
of moles though, this equation also tells us that 2 moles H2 molecules react with 1 mole of O2
molecules to create 2 moles of H2O molecules.

Compound: noun
A substance that is macroscopically homogeneous and that consists of atoms or ions of two or
more different elements in specific proportions.  A compound cannot be separated by physical
means and usually has properties unlike those of its constituent elements.

Mole or mol: noun
A number that is equal to 6.0225 × 1023, or Avogadro's number.  The amount of a substance
that contains as many atoms, molecules, ions, or other elementary units as the number of
atoms in 0.012 kilogram of carbon 12.   Also called a gram molecule.
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