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Moles
What is a chemical mole?
In chemistry a mole is a quantity of
atoms, just like a ream of paper is
a quantity of sheets of paper
A ream of paper is 500 sheets of
Paper.
A mole of atoms is 6.02 x 1023 atoms
That’s 602000000000000000000000 atoms
Why do we need moles?
Atoms and molecules are very small so they
cannot be counted directly. In chemistry we
usually weigh out elements and compounds.
However, simply weighing something does not
tell us how much there is (i.e. how many atoms
or molecules).
For example lead atoms are much bigger than
hydrogen atoms, so 5g of hydrogen would
contain far more atoms than 5g of lead.
6.02 x 1023 = a big number!
6.02 X 1023 Watermelon Seeds: Would be found
inside a melon slightly larger than the moon.
6.02 X 1023 Pennies: Would make at least 7
stacks that would reach the moon.
6.02 X 1023 Blood Cells: Would be more than the
total number of blood cells found in every human
on earth.
602000000000000000000000
A special number
Also known as Avagadro’s number (constant)
Is the number of atoms in 1 mole
of a substance.
But also:
It is the number of atoms in
exactly 12g of carbon
Amedeo Avogadro
Moles are related to Relative Atomic mass
Carbon has a relative atomic mass of
12
As we weigh things in grams we say that in 12g of
carbon atoms there is 1 mole of carbon atoms
Carbon
Relative atomic mass = 12
1 mole of carbon atoms weighs1 12g
The relative atomic mass of hydrogen is 1
Therefore a hydrogen atom is 12 times lighter than a
carbon atom.
Hydrogen
Relative atomic mass = 1
1 mole of hydrogen atoms weighs 1g
Hydrogen relative atomic mass =1
1 mole = 1g which contains 6 x 1023 atoms
Carbon, relative atomic mass = 12
1 mole = 12g which contains 6 x 1023 atoms
Oxygen, relative atomic mass = 16
1 mole =
16g
which contains 6 x 1023
Potassium, relative atomic mass =
1 mole =
39g
39
which contains 6 x 1023 atoms
Arsenic, relative atomic mass =
1 mole = 75g
atoms
75
23
which contains 6 x 10
atoms
What is the mass of:
1 mole of nitrogen atoms? 14g
2 moles of oxygen atoms? 32g
0.5 moles of copper atoms? 32g
How many moles are there in:
11g of Boron atoms?
1 mole
3g of carbon atoms?
0.25 moles
80g of calcium atoms? 2 moles
Useful equation!
Number of moles =
Mass of element (how much you have)
Relative atomic mass of the element
For example. How many moles are there in 4 g of calcium?
Number of moles =
4
40
= 0.1
Magic triangle
MASS
How much you have in g
MOLES
The number of
moles you have of
the substance
FORMULA
MASS
(Relative atomic mass
(atoms) or relative
formula mass (molecules
and compounds)
Moles are not just for atoms!
You can also have a mole of a molecule or a compound.
Diatomic molecules
An oxygen molecule contains 2 atoms of oxygen
16
O
16
O
The formula mass of an oxygen molecule is 32
1 mole of oxygen molecules weighs
32g
1 mole of oxygen molecules contains 6.02 x 1023
oxygen molecules
How many moles in:
28 g of nitrogen molecules?
80g of bromine molecules?
5g of hydrogen molecules?
1 mole
0.5 moles
2.5 moles
What is the weight of:
70g
1 mole of chlorine molecules?
16g
½ mole of oxygen molecules?
2 moles of ammonia (NH3) molecules?
34g
Compounds are just like molecules!
1 mol of a compound is the Relative Formula Mass in grams
Example: methane (CH4)
1
1
H
Relative formula mass =
16
1
H
12
H
C
1
H
Mass of 1 mol of methane = 16
1 mol of methane contains
molecules
6.02 x 1023
methane
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