M l and
Moles
d Masses
M
A mole is 6.022x1023 of anything; atoms, molecules, or even
ppotato chips.
p While this is an incrediblyy large
g number, it is a useful
number when you are dealing with atoms or molecules, because it
represents roughly a handful of a solid material that you might use
in an experiment.
experiment
Since doing chemistry by handfuls of material is inexact, messy,
and dangerous, the preferred chemical technique is to measure all
your chemicals
h i l in
i grams, andd then
th use mathematical
th
ti l conversions
i
to
t
find out how many moles of material you had. Since you should
already have conversions down pat, that means we aren’t really
doing anything new, all we are doing is playing with a different set
of conversion factors.
M l and
Moles
d Masses
M
Each element has a different number of protons and neutrons in
its nucleus, so one mole of each element will have a different mass.
The atomic mass of an element is defined as the mass of one mole
of that element.
You do not have to memorize these masses for my class.
class On any
test you will be given a periodic table that has the atomic mass for
each element listed. I’m going to guess, however, that by the end
off the
th year you will
ill have
h
six
i or eight
i ht off the
th mostt common elements
l
t
memorized anyway, just because you will have used them so many
times.
G
Grams
off elements
l
t to
t moles
l
Problem: I have 50.00 grams of lead. How many moles is this?
((I will treat this jjust like another unit conversion problem)
p
)
Starting units: grams
Ending units: moles
Conversion factor: The atomic mass of lead (Pb)
look it up on the periodic table
At i mass = 207.2
Atomic
207 2 g Pb = 1 mole
l Pb
Set up equation:
grams x (moles/grams) = moles
Plug in numbers and solve:
50.00 g Pb x (1 mole Pb/207.2g Pb) = .2413 moles of Pb
M l off element
Moles
l
t to
t grams
Problem: I want to use .45 moles of carbon in a chemical
reaction. How manyy ggrams of carbon must I use?
Starting units: moles
Ending units: grams
Conversion factor: The atomic mass of carbon (C)
look it up on the periodic table
At i mass = 12.01
Atomic
12 01 g C = 1 mole
l C
Set up equation:
mole x (grams/moles) = grams
Plug in numbers and solve:
.45 moles C x (12.01 g C/1 mole C) = 5.4 g of C
More complicated molar
conversion
i
Now let’s try a more complicated problem. I have 5.00
ppounds of copper
pp that I want to recycle.
y
How many
y moles
of copper is this?
Starting Unit: 5.00
5 00 lbs Cu
Ending Unit: moles of Cu
Conversion factors needed:
1 lbs = 453.59g
1 mole Cu = 63.55 g (atomic mass)
How many moles is 5 lbs of
copper??
Conversion equation:
g of Cu mole of Cu
lbs of Cu ×
×
lbs of Cu
g of Cu
Plug in the numbers:
45359
. g off Cu 1 mole off Cu
5.00 lbs
lb off Cu
C ×
×
1 lbs of Cu
6355
. g of Cu
= 35.6879 moles of Cu
= 35.7 mole of Cu rounded to 3 sigfig
M l l masses
Molecular
Since a molecule is a collection of atoms that are held together
by chemical bonds, the molar mass of a molecule is simply the
sum of the atomic masses of all the atoms that make up the
molecule.
What is the molar mass of water (H2O)?
Atoms in the molecule
2 hydrogens, atomic mass 1.008g
1 oxygen, atomic mass 16.00 g
Total = 2(1.008) + 16.00 = 18.016g
M l l masses
Molecular
Try to get the molar mass of these molecules
C2H6O (ethanol)
CH3CH2OH
(Hint: this is the same molecule, just written a different way)
H2SO4
(NH4)2SO4
(Hint: what does the subscript on the parenthesis mean?)
CoC2O4.2H2O
(Hint: the .2H2O means that two water molecules are part of the complete
molecule and must be included in the molar mass)
M l l masses
Molecular
C2H6O
2(12.01) + 6(1.008) + 16.00 = 46.068 g
CH3CH2OH
H2SO4
2(12.01) + 6(1.008) + 16.00 = 46.068 g
2(1.008) + 32.07 + 4(16.00) = 98.086 g
(NH4)2SO4
2(14.01) + 8(1.008) + 32.07 + 4(16.00) = 132.154 g
CoC2O4.2H2O
58.93 + 2(12.01) + 4(16.00) + 4(1.008) + 2(16.00) = 182.982g
C bi ti Problem
Combination
P bl
Let’s try a problem that combines all the elements you have
learned so far in the tutorials.
If I need 0.50 moles of sodium sulfate for an experiment,
how manyy ounces of this material will I have to weigh
g
out?
C bi ti Problem
Combination
P bl
If I need 0.50 moles of sodium sulfate for an experiment,
how many ounces of this material will I have to weight
out?
y
This problem
p
has several elements combined in it.
Analysis:
Let’s see if you can sort them out. Let’s start by identifying
what you are given and what you need for an answer.
Given: 0.50 moles of sodium sulfate
Answer you want to find: ounces of sodium sulfate
C bi ti Problem
Combination
P bl
Simply put:
0.50 moles of sodium sulfate » oz. of sodium sulfate
So you are given moles and you want to change it to a mass.
How do you do this? First we need a molecular mass
conversion
i factor
f t to
t go from
f
moles
l to
t grams, andd that
th t brings
bi
us to the first part of the problem; you don’t have a
molecular formula, you have a name. So your first step will
be to use the name to come up with the correct molecular
formula.
C bi ti Problem
Combination
P bl
Simply put:
0.50 moles of sodium sulfate » oz. of sodium sulfate
Once you have the correct molecular formula, then the next
stepp is to find the molar mass so you
y can do your
y
mole-to-gg
conversion.
Notice that this will leave us with units of grams,
grams so it
becomes clear that the last part of the problem is converting
from grams to ounces.
C bi ti Problem
Combination
P bl
So let’s put it all together
1. Name to formula
sodium sulfate = Na2SO4
2. Formula to molecular mass
2(22.99) + 32.07 + 4(16.00) = 142.05g
3. Mole-to-g
g conversion
0.50 moles x (142.05g/mole) = 71.025g
4 g-to-oz
4.
g to oz conversion
71.025 g x (1 lbs/453.59g) x (16 oz/1 lbs) =2.50535 oz
C bi ti Problem
Combination
P bl
Actually the preferred method is to put all the calculations into one
long equation so you don’t have to round off between steps:
0 50 mole x (142.05g/1
0.50
(142 05g/1 mole) x (1 lbs/453.59g)
lbs/453 59g) x (16 oz/1 lbs)
=2.50535 oz
And then, only at the end of the calculation, round off to the correct
number of sig figs, 2, and put the correct label on the answer.
=2.5 ounces of sodium sulfate