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Department
of Lehigh Carbon Community College in Schnecksville, Pennsylvania which is just
outside of Allentown.
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Today we will look at the
unit of measure in chemistry called the mole,
and how it relates to Avogadro's number,
and why it is so important when learning chemistry.
Let's start by taking a look at the simple chemical formula you see before you:
Sodium
will combine with chlorine
and will make sodium chloride,
which is common table salt.
By looking at this equation,
you might assume that
if you had one gram
of sodium
and you combine it with one gram of chlorine
that you would make
two grams of sodium chloride.
Conservation of mass; one plus one, your sodium chloride would weight 2.
But this would be wrong
The chemical equation you see before you
does not pertain directly to weights;
it pertains to atoms.
The formula really tells you one atom of sodium
will react what one atom of chlorine
to form one molecule of sodium chloride.
And sodium and chlorine weigh different amounts.
The atomic mass of sodium
is about
23
and the units of atomic mass
are AMU, atomic mass units.
And chlorine
is about 35
AMUs for each atom.
So if you
react these two particles together, two elements,
you would make a sodium chlorate molecule
that would, added together here,
weigh
58 AMUs,
atomic mass units.
Now in a practical sense,
day to day use,
we don't go around
picking up one atom and reacting it with another atom
and trying to do that say on
a bench or even, y'know in your kitchen when you're making
something.
And the AMU is a very, very, tiny number; tiny mass. Obviously y'know, an atom
of sodium
y'know, doesn't weigh very much. We're talking
somewhere in the order of 10 to the minus 30 grams.
So you can't really deal with these very, very small numbers.
So
early chemists, a few hundred year - I guess about a hundred fifty years ago or so
tried to figure out a simple way to be able to deal with these chemical
formulas
with units that are more practical for us to use, for example grams.
unity came up with it
monus uh... called them all
is
if you remember sodium was twenty three m use
into
what f_ lee
could come up with a way
to make that lvm you just change it to gramm's
chlorine was about thirty five a m use
uh...
how but if we can get to thirty-five graham said meant
added together you would forward fifty eight grams
sodium chloride
and the way to get from
any m u
up to gramm's for member twenty-three was how much we won
fatima sodium
they found
if he took
six point oh two two times ten to twenty three
sodium atoms
then it would weigh
twenty three grams
and because
all adamson molecules are made up of the same atomic particles
protons neutrons electrons
it turns out
very conveniently
that mail chlorine atom waist thirty five a m u
if you take six point od to two times ten to the twenty third
chlorine atoms
you'll find a way to be thirty five grams
and likewise over here
if you just took one molecule sodium chloride it would waste fifty eight a m
use
but if you took six point of two too
poster twenty three of those molecules
do they would wait fifty eight grams
and so
by year's end
this number which was called ever got gross number
you can
by simply looking at the periodic table looking at the atomic mass units
of the individual adams
if u
took this many you could just take that a_t_m_ u
value of mass
and just make it equal to grams
and so
amal
is essentially a cal
if you have a mole
of sodium chloride
molecules
that means you have six point o_ t_ two times ten twenty three
molecules of
of sodium chloride
and that mall way field that many molecules will weigh fifty eight grams
so you'll often see
uh...
the term
fifty eight grams uh...
per mole
this how much sodium chloride
ways
so-called that the molar mass
one mol sodium chloride weighs fifty eight grams
safety more chlorine
a mole
of chlorine atoms
ways
thirty five grams
animal of sodium atoms more weight twenty-three cramps
so makes a very convenient way to look at uh...
uh... these
values uh... today
such glee
if you wanted but let's take another example
use of this information
let's say we wanted to make some sodium chloride in the lab
very simply
well look
let's say we wanted to make
uh... let's say to sell fifty eight times too
designed in sixteen we wanted to make a hundred and sixteen
gramps sodium chloride
how do we do it
you know how much sodium and how much chlorine do we need
but we know that
sodium chloride
if we had one more
of sodium chloride
it would weigh fifty-eight cramps
but we want a hundred and sixteen th and cuz i used to write number
socially means we need to mls
are were planning to make two miles of sodium chlorite
to make two molds asorio chloride you'll see a calc lation big to hear we wanna
make to sony and chlorine molecules uh... or moles
sodium chloride
then we're gonna need
to sodium ps
into corinn's
in order to make this uh... compact
himself
we need to moles of sodium
we know that
two moles we need
the sorting out
and we know that
there are
twenty three grams in one mol
suddenly three need forty six grams
sodium
in order to
uh... make this table salt
and since we need to moles
of chlorine
thank loraine ways thirty five grams
for more
we need seventy grams
of chlorine
if he had seventy grams of chlorine with
forty six grams
uh... sodium
you'll see
conservation of mass you get a hundred and sixteen grams over here
of sodium chloride
so it's
use in the camel cool
equation in this
uh... unit of measure the mole
we can deal with every day uh... values or or volume said masses of unit of
uh...
adamson molecules
uh... if we wanted to make something in the lab like we said here we want to
make about a hundred sixteen grams a
table salt
we would need
forty six grams of sodium and seventy grams of chlorine
in order to do that