Answer 1:
What is Avogadro’s number? Why is it a
constant?
Avogadro’s number is a unit of how many of
something you have. This number is very similar to
the way the word “dozen” is used. If I have a
dozen eggs, I have twelve eggs. If you have a
dozen doughnuts, you have twelve doughnuts. In
this case, the word “dozen” isn’t specially
related to eggs or doughnuts, it’s just a way of
counting how many I have. And no matter what the
thing is you’re counting, a dozen is always equal
to exactly twelve of those things. Dozen is a word
used to describe a group of twelve and only twelve
things; when referring to dozens, you could say
that the number twelve is constant.
Similarly, Avogadro’s number is a constant
number. Only instead of counting twelve things in
a dozen, you count an Avogadro’s number (6.022 x
10^{23}) of things in a mole. If I have a
mole of
eggs, I have 6.022 x 10^{23} eggs. If you
have a mole
of doughnuts, you have 6.022 x 10^{23}
doughnuts and
should probably go on a diet. The same goes for
hydrogen and oxygen. If you could somehow count
all of the H_{2} molecules individually in
a mole of
hydrogen, you would count 6.022 x 10^{23}
of them,
and if you could count the O_{2} molecules
in a mole
of oxygen individually you would also find 6.022 x
10^{23} of them.
So Avogadro’s number is just that. It’s just a
number, nothing else.
What’s so special about Avogadro’s number?
Avogadro’s number comes from two important
observations: one involving mass and the other
involving chemistry.
Most of the mass of an atom comes from the
nucleus, which is made up of protons and neutrons.
Protons and neutrons have roughly the same mass,
and this mass is the atomic mass unit (AMU), which
is equal to about 1.6605 x 10^{‐24}
grams.*
Atoms, in general, don’t care about mass when
reacting; atoms care about electrons and bonding.
Sodium doesn’t care that chlorine has an atomic
mass of 35.45 when forming NaCl, it cares that
chlorine has seven electrons in its outer shell.
Whether dealing with a light element like fluorine
or a heavy one like iodine, sodium will always
bond one to one with a group VII halogen because
their electronic structures all look similar.
Unfortunately, it’s very hard to count
individual atoms (who has the time?) but it’s very
easy to measure mass. Since atoms have mass and
the number of atoms you
have should be proportional to mass, then if you
give me the mass of a compound, I can tell you
exactly how many atoms are in it!
There are many ways one could convert between
number of atoms and mass, but a convenient one is
to relate the AMU to grams. How many AMU are in
one gram?
1 AMU/
(1 gram × 1.6605×10^{24} grams) =
6.022×10^{23}
AMU = 1 mole AMU
Avogadro’s number relates atomic masses to
grams. If you have one mole of
hydrogen, which has an atomic number of 1.008 AMU,
then your hydrogen will have a mass of 1.008
grams. If I have 22.99 grams of sodium, I need at
least 35.45 grams of chlorine to react all of it
to NaCl.
One mole of hydrogen and one mole of oxygen
have the same number of molecules because a mole
is a number of things, not a unit of mass. It
doesn’t tell you “what” or “how much”, only “how
many”.
*The atomic mass unit is technically defined as
one‐twelfth the mass of a carbon atom.
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