Download Notes: Moles

Moles
Atoms are very, very small.

1 atom of hydrogen weighs approximately 1.67 x 10-27 kg.
As a result, it’s not very practical to do chemical reactions by counting out the number of atoms or molecules that
will be reacting, because we’ll be counting for a very long time!
You’ve seen this before, because when working with a large number of objects, it’s frequently handy to use units
that are easier to work with.





2 shoes = 1 pair
12 eggs = 1 dozen
144 pencils = 1 gross
500 sheets of paper = 1 ream
6.02 x 1023 atoms or molecules = 1 mole
The idea behind moles is the same as the idea behind “dozens”, except that the number is much bigger.
Definition:
1 mole = 6.02 x 1023 of anything.

6.02 x 1023 is referred to as “Avogadro’s number” in honor of the dude who first worked with it.
If moles is such a handy number, why haven’t you used it before now?

1 mole of most objects that you work with on a daily basis is very, very large. For example, 1 mole of
M&M’s would cover the continental United States to a depth of 125 km.

Although we could use moles to describe numbers of things that we work with everyday, it’s not really very
practical.
Finding Molar Mass
Molar mass (also called “molecular weight” or “molecular mass”): The weight of one mole of a chemical
compound. The unit is “g/mol”.

For elements, the mass of one mole of atoms is called the “atomic mass” and is found on the periodic table.
How to calculate the molar mass of a compound:

For elements, the molar mass is the same thing as the atomic mass.

For chemical compounds, it’s the sum of the masses of all of the atoms in the molecule.
Example: NaCl
Na:
Cl:
23 grams x 1 atom =
35 grams x 1 atom =
Total:
23 grams/mol
35 grams/mol
58 grams/mol

Solve these:
o
o
o
MgCl2
Fe(OH)2
Be3(PO4)2
For these last two examples, tell them
that the molar mass for compounds
like this is found by multiplying
everything in the parentheses by the
number outside the parentheses.
____g/mol
____g/mol
____g/mol
Mole calculations:
How do you count out a mole of atoms?

You don’t. Even if it were possible to count out individual atoms in a reasonable period of time, the
equipment we have only measures “grams.”

As a result, we need to be able to convert between atoms/molecules, moles, and grams.
Use the diagram below with the t-chart method of doing calculations to help you convert between grams, moles,
and molecules/atoms. (This is on your EOC Periodic Table)
grams

molar
mass
moles
6.20 x 1023
molecules
or atoms
Handy hint: In conversion factors, always write “1” in front of “moles”!
Quick recap of the t-chart method:
1. Make a T
2. Put what you’ve got in the top left
3. Put the units of what you’ve got in the bottom right
4. Put the units of what you want in the top right
5. Put in the conversion factors
6. [If needed, add another step to get to where you’re going]
7. Multiply the stuff on the top together and divide by the stuff on the bottom.
Examples: (Go over the T-chart with the first few):

How many grams are in 2.1 moles of Be?

How many molecules are in 6.3 moles of CH4?

How many molecules are there in 11.1 grams of carbon dioxide?

How many grams are in 4.1 x 1023 molecules of H2O?
Molar Mass Practice Worksheet
Find the molar masses of the following compounds:
1)
NaBr
2)
PbSO4
3)
Ca(OH)2
4)
Na3PO4
5)
(NH4)2CO3
6)
C6H12O6
7)
Fe3(PO4)2
8)
(NH4)2S
9)
Zn(C2H3O2)2
10)
AgF
Mole Calculation Practice Worksheet
Answer the following questions:
1)
How many moles are in 25 grams of water?
2)
How many grams are in 4.5 moles of Li2O?
3)
How many molecules are in 23 moles of oxygen?
4)
How many moles are in 3.4 x 1023 molecules of H2SO4?
5)
How many molecules are in 25 grams of NH3?
6)
How many grams are in 8.2 x 1022 molecules of N2I6?