Transcript 1 mole - My CCSD

THE MOLE
Return to dimensional
analysis
THE MOLE
 What
is a mole?
 Not a furry animal that burrows in
the ground
 What it is? . . . A unit of measure
that tells how much of something
you have
THE MOLE
 Examples:
1
dozen eggs is how many eggs?
 A couple of dollars is how many dollars?
 1 score of people is how many people?
 The
mole is another example of a word
that describes a number of something
THE MOLE
 How


much is a mole?
REMEMBER: In chemistry, we are talking
about atoms and there are a LOT of atoms
Therefore the mole is a VERY BIG number
1
mole of stuff is 6.02 x 1023 items of the
thing

For example if you have 1 mole of pennies
this equals 6.02 x 1023 pennies
THE MOLE
 Where

did we get this number?
6.02 x 1023
 Discovered
by a scientist by the name
of Avagadro.
 Therefore, it is called AVAGADRO’S
NUMBER
 He discovered how many atoms
made up 12 grams of carbon.
 Became the basis for the number of
any number of particles in 1 mole
Just
THE MOLE
how big is the mole?
If you had 1 mole of pennies (6.02 x
1023 pennies), then you could
cover the entire earth with pennies
100 feet deep.
Let’s
look at some other examples
THE MOLE
 Now
that we have the mole as a
measurement, we can now find the
number of particles in a sample
 For example:


1 mole of a compound is 6.02 x 1023
molecules
1 mole of an element is 6.02 x 1023 atoms
EXAMPLE
 If
you have 2 moles of iron, how many iron
atoms do you have? (We will continue to
use dimensional analysis)
1

mole = 6.02 x 1023 atoms
2 moles | 6.02 x 1023 atoms = 1.20 x 1024 atoms
|
1 mole
TRY THESE
1.
If you have 3.5 moles of copper, how
many copper atoms do you have?
2.
If you have 0.0235 moles of water, how
many water molecules do you have?
3.
If you have 3.31 x 1024 atoms of
potassium, how many moles do you
have?
ANSWER
1.
2.11 X 1024 atoms of copper
2.
1.41 x 1022 molecules of water
3.
5.5 moles of potassium
MOLAR MASS
 Yay!
We get to use the periodic table
again.
 We talked earlier in the year about the
mass number


The number located at the bottom of each
element
For example:
 Mass
number of Lithium is 6.94
 Mass number of Bromine is 79.90
MOLAR MASS
Before
the mass number was
the measure of the number of
protons + neutrons
It also accounted for all the
isotopes (that’s why some
numbers have decimals)
MOLAR MASS
 It
turns out the molar mass has
another value
 Molar mass: the mass in grams of
one mole of any pure substance.
 For example
 1 mole of Lithium = 6.94g/mole
 1 mole of Bromine = 79.90g/mole
WHAT HAPPENS WHEN YOU
HAVE A COMPOUND?
How
do we find the molar
mass of water (H2O)?
TRY
IT
SOLUTION
 To
find the molar mass, you add the molar
mass of ALL the elements in the compound
 H2O
 You have 2 H  2 (1.01 g/mole) =
2.02g/mole
 You have 1 O  16.0 g/mole
 Total molar mass = 16.0 g/mole + 2.02
g/mole
 Therefore the molar mass of water is:
 18.02 g/mole H2O
TRY THESE – Be careful of the
subscript numbers
1.
CCl4
2.
NaBr
3.
Pb(SO4)2
4.
AgNO3
SOLUTIONS
1.
2.
3.
4.
153.81 g/mole CCl4
102.89 g/mole NaBr
399.32 g/mole Pb(SO4)2
169.88 g/mole AgNO3
NOTICE: The units for molar mass are
g/mole (this is a conversion unit)
HOW TO USE MOLAR MASS
 We
can now measure the mass of
substances and determine how
many moles and how many
particles we have?
 For example: You measure 22.0g of
boron (B), what is the number of
moles you have? What is the
number of boron atoms you have?
EXAMPLE
 22.0g
 2.04
of B | 1 mole of B = 2.04 moles of B
|10.81 g of B
moles of B | 6.02x1023 atoms of B =
|
1 mole of B
1.23 x 10 24 atoms of B
TRY THESE
1.
2.
If you have 13.8g of lithium,
how many moles is this? How
many atoms?
If you have 88.5g of chlorine,
how many moles is this? How
many atoms?
ANSWERS
1.
1.99 moles; 1.20 x 1024 atoms
of Li
2.
2.50 moles; 1.50 x 1024 atoms
of Cl
FINDING THE MOLES FOR A
COMPOND
 Now
that we know how to find the molar
mass of a compound, we can calculate
the number of moles and the number of
molecules of a compound
 The procedure is just like an element,
except you have to calculate the molar
mass first.
 YOU MUST ALSO KNOW HOW TO NAME
COMPOUNDS
EXAMPLE
 Everyone
try this out:
You start with 261 g of manganese
(IV) oxide. What is the molar mass
of manganese (IV) oxide? How
many moles is this? How many
molecules is this?
SOLUTION
4.
To find the molar mass, you need to use
the periodic table:
manganese (IV) oxide = MnO2
1 Mn = 54.94 g/mole
2 O = 2(16 g/mole) = 32 g/mole
Molar mass = 54.94g/mole + 32
5.
Total molar mass = 86.94 g/mole

1.
2.
3.
g/mole
SOLUTION (CONT)
 Starting
mass: 261g
 Molar mass: 86.94g/mole
261g of MnO2 | 1 mole
=
| 86.49 g of MnO
3.02 moles of MnO2
SOLUTION (CONT)
 3.02
moles | 6.02 molecules =
|
1 mole
1.82 x1024 molecules of MnO2
TRY THESE – You can do
dimensional analysis all at once
1.
2.
You start with 361g of MgSO4.
How many molecules of
magnesium sulfate do you have?
You start with 370.5g of Ca(OH)2.
How many molecules of calcium
hydroxide do you have in this
sample?
SOLUTION
1.
1.81 x 1024 molecules MgSO4
2.
3.010 x 1024 molecules
Ca(OH)2
LET’S BRING IN DENSITY
 We
know that density is a conversion
factor between mass and volume
 Density has the following units:
 Mass/volume
 Grams/milliliter (g/mL)
 Grams/cubic centimeter (g/cm3)
 Kilograms/liter (kg/L)
 etc . . .
DENSITY
 As
long as we know that density is a
conversion factor, now we can convert
number of molecules to volume or
volume to number of molecules

Molecules  Moles  Mass   Volume
(atoms)
avagadro’s #
molar mass
density
EXAMPLE


You have a sample of iron that contains
3.00 x 1025 atoms. If the density of iron is
7.87g/mL, what is the volume that the
iron takes up?
To do this, it’s just like any other
dimensional analysis problem
1.
2.
Write down what you start with
Cancel out the stinking units
EXAMPLE
 Step


1:
You begin with 3.00 x 1025 iron atoms.
What is the next thing we have to get to
head towards volume?
Molecules/Atoms  Moles
 3.00 x 1025 atoms |
1 mole_______
6.02 x 1023 atoms
EXAMPLE
 Step
2
After you get to moles where is the next
stop on your way to volume?
Molecules/Atoms  Moles  Mass
 You need the molar mass of iron (Fe)

 55.85

g/mole
3.00 x 1025 atoms |
1 mole_____| 55.85 g__
| 6.02 x 1023 atom | 1 mole
EXAMPLE
 Step

3
The final conversion we need is density
Molecules/Atoms  Moles  Mass 
Volume


Density of iron is 7.87 g/mL (you will not be
able to do the problem unless density is
given
3.00 x 1025 atoms | 1 mole_____| 55.85 g_| 1mL__
| 6.02 x 1023 atom | 1 mole | 7.87 g
SOLUTION
 353
mL
 NOTICE:


Each time we needed to do a conversion,
we made sure the units cancelled out
You have to follow the correct pathway to
get from the unit you start with to the unit
you are trying to find.
TRY THESE
1.
2.
3.
You start with 48.1 mL of H2O. The
density of water is 1.00g/mL. What is
the number of moles of water you
have?
You have 3.5 moles of mercury. What
volume does this take up if the density
of mercury is 13.6 g/mL?
You start with 9.90 x 1020 molecules of
ethyl alcohol (C2H6O). If the density of
ethyl alcohol is 0.791 g/mL, what
volume does this take up?
SOLUTION
1.
2.67 moles of H2O
2.
52 mL of Hg
3.
0.0958 mL of ethyl alcohol