Moles
A way to
count
atoms or
molecules
How can we count atoms?
Mole
The SI base unit that
describes the amount
of a substance
“like a dozen, a
score, a ream, etc…”
…well, let’s see… how many Carbon atoms
are there, in, say, 12 grams of Carbon?
Answer: there are 6.022 x 1023
We count eggs by the dozen (12 eggs)
that’s 602,000,000,000,000,000,000,000!
We count paper by the ream (500 sheets)
What shall we count atoms by?
…well, let’s count atoms by that huge number
and call it
“a mole” of atoms
We count atoms by the mole (6.022 x 1023)
Say What???
“one mole” is like “one dozen”
1 dozen = 12
For example, if we stack 1 mole of pennies
on top of one another, how tall would the
stack be?
The answer is it would be so tall that the
stack of pennies could reach the sun and
back almost 500 million times!
1 score = 20
1 gross = 144
1 mole = 602,000,000,000,000,000,000,000
or 6.02 x 1023
1
a mole of apples is 6.02 x 1023 apples
1 mole of gummi worms
…one dozen eggs weighs
more than one dozen peanuts
= 6.022 x 1023 gummi worms
1 mole of carbon
1 mole of gumballs is 6.022 x 1023 gumballs
= 6.022 x 1023 carbon atoms
1 mole of paper
1 mole of popcorn is 6.022 x 1023 kernels of popcorn
= 6.022 x 1023 sheets of paper
Who weighs more??? Obviously the gumballs, but the
quantity is the SAME for both!
1 mole of bricks
= 6.022 x
1023
bricks
6.022 x 1023
Where did that number come from?
Why use that number?
Because then the
average atomic mass
will always be equal to
the mass (in grams)
of 1 mole of that element.
This much popcorn would cover the entire US 310
miles high! Whew, that’s a lot of poppin’!
This number on the periodic
table has two meanings;
…it is the
average atomic mass
in amu
it is also the molar mass
of that element; the mass
in grams of 1 mole of that
element
Avogadro’s Number
Molar Mass
…is the mass
in grams of
1 mole of X
…has units of grams per mol
g/mol
Example:
Carbon is 12.011g/mol
6.022 x 1023 is called Avagadro’s number.
It is the number of particles in one mole.
The number comes from
the number of C atoms in
12.011 g C, and was
calculated by Joseph
Loschmidt, but is named
after Count Amadeo
Avogadro,
2
Avogadro’s Number
is very convenient because:
Mole-Gram Conversions
1 mol C = 12.011 g C
1 mole of carbon = 12.011 g C
therefore…
12.011 g C = 1 mol C
1 mole of oxygen = 15.994 g O
1 mole of lithium = 6.941 g Li
1 mole of Magnesium = 24.305 g C
How to convert moles to grams
Looks like a conversion factor
Smells like a conversion factor
It is a conversion factor
Use it for DIMENSIONAL ANALYSIS
Convert moles to grams
Our conversion factor is
1 mol Li = 6.941 g Li
Example:
Convert 1.92 moles of Lithium
to grams of Lithium
How to convert grams to moles
…because the molar mass of Lithium is
6.941 g/mol
So, use a little dimensional analysis:
6.941 g Li
1.92 mol Li x __________
1 mol Li
(
)
= 13.33 g Li
How to convert grams to moles
well, we know that;
1 mole of carbon = 12.011 grams of carbon
Example:
Convert 5.0 grams of Carbon
to moles of Carbon
in other words;
1 mol C = 12.011 g C
So, use a little dimensional analysis:
5.0 g C
x
1 mol C
__________
(
12.011 g C
)
=
0.42
mol C
3
gram – mole conversion:
Avogadro’s Number
is also very convenient because:
we can calculate
how many atoms
are in a sample of matter
for example,
if I have 5 g of carbon
how many atoms of carbon do I have?
How to calculate # of atoms:
Sample Problem:
First calculate the number of moles
5.0 g C
x
1 mol C
__________
(
12.011 g C
)
=
0.42
mol C
Determine the mass in grams of 5.50
moles if iron.
And then use then multiply that by Avogadro’s #:
6.022 x 1023 atoms C
(
0.42 mol C x ______________
1 mol C
)
= asdf mol C
Determine the amount of iron
atoms present in
352 grams of Fe.
4
Practice Problems
1.
How many grams are in 2.50 mols of S?
2.
How many grams are in 3.20 mols of Cu?
3.
How many mols are present in 11 grams of Si?
4.
How many mols are present in 205 g of Hg?
5.
What is the mass in grams of 0.48 mol of Pt?
Solutions!
1.
80.2 g S
2.
203 g Cu
3.
0.39 mol Si
4.
3.1 mol Hg
5.
94 g Pt
5