Transcript The Mole - TeacherWeb

The Mole
Historically, there was no way
to know the
mass of one atom or molecule
WHY?
Because
atoms are so
small!
Experiments by a French chemist,
Joseph Louis Gay-Lussac opened
the door to making determining
the mass of elements possible.
Volumes of Gases and the mole
The experiment: equal volumes of gases
at the same temperature and pressure
reacted and the following occurred:
A
A
½A
+
+
+
B
½B
B
→
Solid forms
→
→
½A
½B
+ solid
+ solid
Gay-Lussac’s law
• “The law of combining volumes states that, when gases
react together to form other gases, and all volumes are
measured at the same temperature and pressure:
The ratio between the volumes of the reactant gases and
the products can be expressed in simple whole
numbers”
In other words: 2H2 + O2 → 2 H2O
coefficients are whole number ratios and can express
volume as well as numbers of atoms/molecules
• Based on these results, what can
be concluded about the number
of molecules in equal volumes of
gases measured at the same
temperature and pressure??
Along came a guy named Amedeo
Avogadro
Amedeo Avogadro determined that equal volumes of gases
at the same temperature and pressure contain the same
number of molecules/atoms. He himself did not determine the
number of atoms in a mole, but his principle of gas volumes
led to its discovery. This is known as Avogadro’s Hypothesis.
Why was this significant? If equal volumes had
equal numbers of molecules, then you can
compare the masses of equal volumes in a
ratio.
Mass 1 liter oxygen =
Mass 1 liter hydrogen
16
1
Mass 1 liter nitrogen = 14
Mass 1 liter hydrogen
1
What would happen if you
compared 2 Liters of each gas?
Mass 2 liter oxygen =
Mass 2 liter hydrogen
?
= 16
1
Mass 2 liter nitrogen =
Mass 2 liter hydrogen
=14
1
?
What would the ratio of one atom of
oxygen to hydrogen be?
16 to 1!
What would the ratio of one atom of nitrogen
to hydrogen be?
14 to 1
This enabled scientists to standardize
these relationships - atomic mass units
If H = 1 amu
Then:
• O = 16 amu
• N = 14 amu
• Look on the periodic table – the masses
given on the table for each type of atom is
given in amus
So what does this have to do with
the mole???
• Remember: atoms are small
• Scientists wanted to determine the number of
atoms in a standard amount of substances.
Originally they used hydrogen, then oxygen and
finally carbon.
• They experimentally determined the number of
atoms in 12.0g of carbon
• They called this a mole (molecular weight)
How many atoms are in 1 mole???
• You guessed it: 6.02 x 10 23
• It is the number of particles (atoms) in 12.0
g Carbon
This is a very large number!
Consider this:
One mole of paper clips strung together
would wrap around the world 400 trillion
times!
More mole facts
• 6.02 X 1023 Watermelon Seeds: Would be
found inside a melon slightly larger than
the moon.
• 6.02 X 1023 Donut Holes: Would cover the
earth and be 5 miles (8 km) deep.
• 6.02 X 1023 Pennies: Would make seven
stacks that would reach the moon.
A Mole is a shorthand way of
saying 6.02x 1023
• When you go to a bakery and ask for a
dozen donuts, how many donuts do you
get?
• If I asked you to sit in pairs, how many
students would sit together?
• “Pairs” and “dozens” are examples of
counting numbers that you are familiar
with and that you can easily count.
• A mole is known as a “chemist’s dozen”
So how can we easily find out the
mass of one mole of anything?
• We use the periodic
table
• The atomic mass of
an element is not only
equal to the mass in
amus of one atom of
the element, but also
the mass in grams of
one mole of that
element
Now all molecular weights are
based on carbon = 12 amu
6
C
12.0
12.0 amu is the mass of one atom
of carbon or the mass in grams of
one mole of Carbon atoms
An amu is defined as 1/12 the
mass of a carbon-12 atom
1amu = 1.66 x10-24g
OR
1 gram = 6.02x 1023 amus
Going back to the balloons
• If I have 32.0g of O2 and 2.0 g H2, what could I
say about the volumes they take up?
• They must be the same….
• If a substance is in the gas state at 1.00 atm and
0oC, then one mole of that substance takes up a
standard volume of 22.4L
– This is the molar volume at STP (standard
temperature and pressure)
Therefore:
• 1 mole H2 = 6.02 x 1023 molecules H2 = 2.02 g H2 = 22.4L
• 1 mole O2 = 6.02 x 1023 molecules O2 = 32.0 g O2 = 22.4L
• 1 mole C = 6.02 x 1023 atoms C = 12.0 g C
• 1 mole H2O = 6.02 x 1023 molecules H2O= 18.0 g H2O =
22.4L
Where did I get the 18.0 g from?
Look at your periodic table
• 2 x 1 mole H = 2 (1.01g H) = 2.02 g H
• 1 x 1 mole O = 1 (16.0 g O) = 16.0 g O
= 18.0 g H2O
Mole relationships
1 MOLE
6.02 x 1023
particles
MOLAR
MASS
22.4L if a
gas at
STP
Use the periodic table to find the
mass in grams of 1 mole of
aluminum atoms
• 1 mole Al = 27.0 g Al
• What is the mass of two (2) moles of aluminum?
• USE Dimensional Analysis!!
2 moles Al x 27.0 grams Al = 54.0 grams Al
1 mole Al
• Can you figure out the mass of one mole of
carbon dioxide (CO2)?
•1 mole of C = 12.0 g
•1 mole of O = 16.0 g
•What is the mass of one mole
of CO2?
C = 1 X 12.01g = 12.0g
O = 2 X 16.00g = 32.0g
44.0g
The molar mass of CO2 = 44.0g
How about 1.50 moles CO2?
• 1 mole CO2 = 44.0g CO2
• 1.50 mole CO2 x 44.0g CO2 = 66.0 g CO2
1 mole
67.0 g H2O = ? moles
• 1 mole H2O = 18.0 gH2O
• 67.0 g H2O x 1 mole H2O= 3.72 moles
18.0 gH2O
• How many molecules H2O are in 3.50
moles H2O?
• 1 mole H2O = 6.02 x 1023 molecules
• 3.50 moles H2O x 6.02 x 1023 molecules
1 mole H2O
= 2.11 x 1024 molecules H2O
How many atoms of H are in 3.45
moles of H2O?
• Two approaches:
1. How many moles of H are in 1 mole of H2O?
3.45 moles H2O x 2 mole H
1 mole H2O
x 6.02 x 1023 atoms H
1 mole H
OR
2. How many atoms of H are in one molecule H2O?
3.45 moles H2O x 6.02 x 1023 molecules H2O x 2 atoms H
1 mole H2O
1 molec H2O
So that is the story of the mole!
• Now we will try some
problems in class