Using Chemical
Formulas
Definitions

The Mole


The amount of a substance that contains as many
particles as there are atoms in exactly 12g of
carbon-12.
Avogadro’s Number


The number of particles in exactly one mole of a
pure substance.
6.022 x 1023
Understanding the Formula

Relationship between the number of moles of
each atom in a chemical compound

H2SO4



2 moles of Hydrogen
1 mole of Sulfur
4 moles of Oxygen
Molar Mass

The mass of one mole of a pure
substance.

1 mole Water = 18.00g

1 mole ammonia = 17.00g

1 mole Carbon Dioxide = 44.00g
Molar Mass

Molar Mass Calculation for Sulfuric Acid
(H2SO4)

Make a list of each element and the number of
atoms of each element present in the
substance.



H2
S1
O4
Molar Mass

Go to periodic table and determine the atomic
mass average (atomic weight) of each
element.



H 1.00794 g/mol
S 32.066 g/mol
O 15.9994 g/mol
Molar Mass

Multiply each atomic mass by the number of
atoms in the formula.



H 1.00794 * 2 = 2.015 g/mol
S 32.066 * 1 = 32.066 g/mol
O 15.9994 * 4 = 63.998 g/mol
Molar Mass

Add up the results of step three:

2.015+32.066+63.998 = 98.079 g/mol = molar mass
of sulfuric acid
Molar Mass Calculations

The Four Steps to Solving Mole Problems:

Step 1: Figure out how many parts your
calculation will have by using the diagram
Molar Mass Calculations

Step 2: Make a T-chart, and put whatever
information the problem gave you in the top
left. After that, put the units of whatever you
were given in the bottom right of the T, and
the units of what you want to find in the top
right.
Molar Mass Calculations

Step 3: Put the conversion factors into the Tchart in front of the units on the right.

As we saw from the "map", the conversion factor between grams and
moles is the atomic mass of copper. Because we measure atomic mass
in grams, you need to put the atomic mass in front of the unit "grams of
copper". What do you put in front of moles? Whenever you do a
calculation of this kind, you need to put "1" in front of moles, like you see
here:
Molar Mass Calculations

Step 4: Cancel out the units from the top left
and bottom right, then find the answer by
multiplying all the stuff on the top together
and dividing it by the stuff on the bottom.

In this case, you'd multiply 22 by one and divide the result by 63.5. Your
answer, 0.35 moles of copper:
Molar Mass Calculations


Solving Two-Step Mole Calculation
Problems:
What happens if we need to solve a problem
that requires we not just go from one box in
the next in our diagram, but across the entire
diagram?
Molar Mass Calculations

How many atoms are in 22 grams of copper?

Instead of doing a simple one step calculation, we
need to do a two-step calculation, with the first
step going from grams to moles and the second
step going from moles to atoms.
Molar Mass Calculations

How can we solve this kind of problem?

We start off by doing the same thing that we did in
our last example: We had to convert grams to
moles before, and we can see from the map that
we have to convert grams to moles now, too. To
refresh your memory, here's the calculation from
last time:
Molar Mass Calculations




In the next step, we do the same thing over again, except that we
need to add another T to the T-chart.
When you do this, take the units of the thing at the new top left and
put them on the bottom right (in this case, moles).
Then take the units of what you want (in this case, atoms) and put it
in the top right.
Finally, put in your conversion factors, which from the chart above is
Avogadro's number, or 6.022 x 1023. Since this number refers to the
number of atoms in a mole of a substance, we put this in front of
"atoms of copper". Again, put the number "1" in front of moles,
because we're saying that there are 6.022 x 1023 atoms in ONE
mole of an element.
Molar Mass Calculations

When we add all these terms in, we can
cross out the units that cancel out, as shown.
To get the answer, multiply all the numbers
on the top together and divide by the
numbers on the bottom. Your answer should
then be set up like this: