A VICTORIA SCIENCE TEACHING RESOURCE
Avogadro’s Number
6.022 x 1023 or 602,200,000,000,000,000,000,000
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WHAT IS IT?
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WHAT DOES IT
MEAN?
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WHAT’S A MOLE?
Avogadro’s Number is the number of atoms,
molecules, or other objects that makes up one mole of
a substance. For example:
6.022 x 1023 hydrogen atoms represent one mole of
hydrogen.
6.022 x 1023 water molecules represent one mole of
water.
6.022 x 1023 elephants represent one mole of elephants.
Using Moles and Avogadro’s Number
Take this reaction as an example:
NaOH + HCl = NaCl + H2O.
Because this looks like an even reaction on either side,
it would be easy to assume that you can add equal
weights of NaOH and HCl together and have them
all used up in the reaction. But the elements that
make those molecules up have different densities, or
as scientists say their ‘atomic mass’ is different. For
example, you can’t add one cup of sodium hydroxide
and one of hydrochloric acid to get a complete
reaction. This is because if you measure out the same
volume of these chemicals you’ll have more molecules
of the dense one and there won’t be an exact number
to react and your end result will not be what you
expect.
That’s where Avogadro’s number comes in. It is a really
useful way to standardise the way we predict and carry
out chemical reactions, in the same way that working
in teaspoons, grams or millilitres allows us to mix a
perfect sponge cake. Once chemists have worked out
how much a mole of a chemical weighs they can always
ensure that they have approximately the right number
of molecules to make a reaction work fully.
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WHY IS THAT A
PROBLEM?
The mole is the unit of measurement for the amount
of a substance as set out by the International System
of Units. Other SUI units of measurement include
kilograms, metres, and seconds. It’s a very handy way
for chemists to work with huge numbers of atoms or
molecules.
AVOGADRO’S NUMBER
Avogadro didn’t discover the number himself— it was
named after him in recognition of his contributions to
chemistry. Avogadro’s number helps chemists to get
around the problem that every element has a slightly
different mass, or density.
