[MUSIC]
Lighthouse Scientific
Education presents
a lecture in the Mole Series.
The topic; Atomic Mass
and Molecular Weight
The similar terms
formula weight and
molar mass are also discussed.
Material in this lecture
relies on an understanding
of the previous lectures
The Mole as a Quantity
and from the Measurement
and Number Series,
Scientific Notation
The lecture begins with
definitions and principles
for atomic molar mass,
and molecular weight,
formula weight and molar mass.
A set of basic rule are
covered for determining
the these weights or mass
3 practice problems are given.
To definitions. Molar
Mass is the mass of one mole
of an element or compound.
It is given as mass per
mole and the units are
g per mole.
In the previous lecture
it was shown that the mole
is just a number
(a counting unit).
It is a very large number
because atomic scale
particles are very small.
It is called Avogadro's number
and is usually written
in scientific notation.
Often just the first 3
numbers of the coefficient,
6.02, are used.
Atomic molar mass is the
molar mass of an element.
It is the weight of a mole
of a single type of atom.
Molecular weight,
abbreviated Mr or MW
is the sum of the atomic
molar masses of each atom
in a molecule or
polyatomic ion.
It is very similar to
molar mass and two terms
are somewhat interchangeable.
Molecular weight is the
more commonly used term.
Formula weight is the sum
of the atomic masses from
a formula (molecular
weight is a formula weight).
Formula weight is a
bit broader because
it includes ionic compounds.
For a molecule, any of the
three terms, molar mass,
molecular mass or
formula mass can be
considered correct.
Use the one that
your instructor favors.
We'll be favoring
molecular weight.
Let's dig a bit deeper
with atomic molar mass.
It is the molar
mass of an element
It includes the
relative abundances of
the elements isotopes.
That is, for a single
element the different isotopes
that its atoms come in.
These are in
specific proportions.
The atomic mass of an
element is Avogadro's
number of atoms
in those regular or
specific proportions.
To make matters easier,
the atomic molar mass is
given as the mass number
of the element on
the Periodic Table.
Calling up a Periodic
Table and taking
a closer look at the
element carbon shows
the atomic number written
at the top of the symbol
and the mass number,
at the bottom.
The atomic molar
mass of carbon is
12.01 g per mole.
A look at nickel, its
mass number is 58.69.
The atomic molar
mass of nickel is
58.69 g per mole.
Silicon has a mass
numberof 28.086
The atomic molar mass of
silicon is 28.086 g per mole.
Getting atomic molar mass is
a straight forward process.
Molecular weight
of a compound is a
bit more involved but
it can be broken down
into relatively simple steps.
The rules are given
for molecular weight but
they also work for formula
weight and molar mass.
As mentioned, molecular
weight is the sum of the
atomic masses of each
element in the molecule
or polyatomic ion.
It is found by getting,
and adding, all of the
atomic molar masses
of ALL OF THE ATOMS
in the formula.
For example, the
molecular weight of H20 is
the sum of the atomic mass
of 2 hydrogen and 1 oxygen.
The molecular weight
of ammonia, NH3 is
the sum of 1 nitrogen and
3 hydrogen atomic masses.
And the molecular weight
of the sugar glucose,
C6 H12 O6, is the sum
of 6 carbon, 12 hydrogen
and 6 oxygen atomic masses.
Determining the molecular
weight of a compound
is an essential skill
moving forward in the
study of chemistry.
We should therefore consider,
and practice, the process.
A basic 4 step process
can be constructed that
yields a molecular weight.
It begins with a
chemical formula.
What is the compound?
What atoms is it composed of?
What is the ratio of the
atoms in the compound?
These are some
basic questions asked
and answer in this step.
From there get the
atomic molar mass for each
element in the compound.
Using the formula and
the atomic molar masses,
multiple the subscript
for each element in the
formula by its atomic mass.
If there are 2 hydrogens
in the formula multiply
the atomic mass
of hydrogen by 2.
Finally, add together all of
the masses calculated in step 3.
That value will be
the molecular weight.
Most students master
this process after solving
for a couple molecular
weights on their own.
Let's put the rules to
work with a first example.
The formula is that forwater;
H 2 O. The second step
says to get the atomic
molar mass for each element.
Calling up a Periodic Table,
the atomic molar mass
for hydrogen is
1.008 g per mole.
Write that down.
The atomic molar mass for
oxygen is 16.00 g per mole.
Write that down.
Step 3 has us take the
atomic molar mass
for each element, we'll
start with the hydrogen,
and multiple it by its
subscripts in the formula.
Hydrogen's subscript
in the formula is a 2.
That gets a value of 2.016 g
of hydrogen per mole of water.
For oxygen its atomic
mass is multiplied
by 1 since its subscript
is a 1 in the formula.
As we know when the
subscript in the formula
is 1 it is not written
but instead implied.
Additionally, there is
no need to multiple by 1
because it does
not alter the value.
There are 16.00 g of
oxygen in a mole of water.
These are the
values needed for the
addition in step 4.
Write out the addition and sum.
The molecular weight of
H2O is 18.016 gram per mole.
That is 1 mole of water
has a mass of 18.016 grams.
One final note
needs to be made.
Since addition was done
with values that have
different significant figures
a check is needed to make
sure the final answer to
the process has itself the
correct number of
significant figures.
Not all instructors are
stickler for this point
but we will treat
them like they are.
First to the hydrogen.
Its last significant figure
is in the thousandth position.
After the decimal point
it goes tenths, hundredths,
thousandths. The oxygen,
the final sig. fig.
in its atomic mass
is in the hundredths.
The rules for adding
or subtracting numbers
has the answer of the math
having significant figures
the match the starting value
with the least amount of
information in its value.
That would be the oxygen
since it does not have
information in the thousandths.
This means that the molecular
weight of water can only
be given to the hundredths
position like the
atomic molar mass oxygen.
The 6 in the thousandths
position still plays
a role in the rounding
of the 1 in the hundredths.
Since 6 is larger or equal
to 5 the 1 is
rounded up to a 2.
The molecular weight of
H2O, as calculated here,
is 18.02 g of water
per mole of water.
For those who need it there
is an entire lecture on
significant figures
in the Measurement
and Number series.
Example 2 and the formula
is that of the ion carbonate.
C O3 with a negative 2 charge.
This example will show that
molecular weight of an ion
are determined the same
way as any other compound.
That carbonate has
an extra 2 electrons
but they do not affect
the molecular weight
since electrons
have a tiny fraction
of the mass of the other
sub atomic particles and
therefore are treated
as insignificant
contributors to
overall all mass.
Step 2, get the atomic
molar mass for each element.
Calling up the Periodic Table;
starting with carbon
and its atomic mass
of 12.01 g per mole.
Keep track of that.
Then to oxygen,
its atomic molar mass
is 16.00 g per mole.
Keep track of that.
Step 3 has us take
the atomic molar masses
and multiply them by
the element subscripts.
For carbon that is
the 12.01 g per mole
times the subscript
of the implied 1.
That math is easy.
There are 12.01 g of carbon
in 1 mole of carbonate ion.
For the oxygen it
atomic mass is multiplied
by the 3 in the
chemical formula.
That gives oxygen's
contribution as 48.00 g.
What about the charge?
Nowhere does that -2 charge
come into play in getting
the molecular weight.
These are the values that
are added together to
get the molecular weight.
12.01 plus 48.00 gives
a value of 60.01 g.
60.01 g of carbonate is the
weight of 1 mole of carbonate.
A quick check of sig.
figs. shows both the
atomic molar mass of carbon
and oxygen to go to the
hundreds place.
As does the calculated
molecular weight.
It has the correct number
of significant figures.
Example 3 is a larger molecule.
It is a sugar molecule
with a general formula
of C 6 H 12 O6.
In step 2 we get the atomic
molar mass for each element.
Using the Periodic
Table, carbon as we might
remember has an atomic molar
mass of 12.01 g per mole.
Hydrogen has an
atomicmolar mass
of 1.008 g per mole
and the atomic molar mass of
oxygen is 16.00 g per mole.
For step 3 these
values are multiplied by
their respective subscripts.
Carbon's atomic molar mass
is multiplied by
the subscript 6 for
a mass contribution
of 72.06 grams.
Hydrogen's atomic
molar mass is multiplied
by its subscript of 12.
That math
yields a mass contribution
of 12.096 grams.
And finally, the 16.00 g of
oxygen is multiplied by 6.
16.00 g times 6 is 96.00 grams.
There is 96.00 g of oxygen in
1 mole of the sugar molecule.
Take these three
values and add them up.
72.06 plus 12.096
plus 96.00 equals
180.156 There are 180.156 g
of the sugar in
a mole of sugar.
And sig. figs. ? Both
the carbon and the oxygen
have atomic molar masses
given to the hundredths.
The final answer
must therefore be
rounded to the hundredths.
Does the 5 in the hundredths
stay a 5 or should
it be rounded to a 6?
That 6 that follows the 5
says round up for a final
molecular weight of
180.16 g per mole.
Another way to handle the
sig. fig. issue is to find the
atomic weight of oxygen
and carbon that go to the
thousandths position
like the hydrogen.
To recap the lecture
Molar Mass is the mass
of one mole of an element
or compound.
The units are g per mole.
A mole is a number; 6.022
times 10 to the 23rd
Atomic molar mass is the
molar mass of an element.
It is the mass of one
mole of an element.
The mass includes
relative abundances
of the element's isotopes.
Importantly, it is given
as the mass number of the
element on the Periodic Table.
The molecular weight (as
well as the formula weight
and the molar mass) of a
compound or polyatomic ion
is found by: having a formula;
getting the molar
mass for each element
in the formula;
multipling each element's
subscript in the formula
by its atomic mass
and finally adding all
the masses calculated
in previous step.
And that concludes the lecture.
Practice getting
molecular weights.
They are an important
tool and then go to
Molecular Weight and
Conversion lecture
to further develop the concept.
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