Do now!
Have a look
through your test!
Moles!
Moles!
Equal masses of different elements will
contain different numbers of atoms (as
atoms of different elements have different
masses)
Moles!
It is sometimes useful for physicists and
chemists (but we don’t care about them) to
compare the number of atoms or molecules
in an amount of substance.
To do this we use the idea of moles.
A chemist
Moles!
You need to learn this
definition.
• One mole of a substance contains the same
number of molecules/atoms as in 12 grams
of carbon-12.
• This number (of atoms or molecules) is
known as the Avogadro constant (NA)
which is equal to 6.02 x 1023
How big is 6 x
23
10 ?
Imagine the whole of the United states
You are here!
How big is 6 x
23
10 ?
Imagine the whole of the United states
covered in unpopped popcorn
How big is 6 x
23
10 ?
Imagine the whole of the United states
covered in unpopped popcorn to a depth of
six miles!
How big is 6 x
23
10 ?
Imagine the whole of the United states
covered in unpopped popcorn to a depth of
six miles! Count the grains and that is 6 x
1023!
600000000000000000000000
Moles!
For example, Hydrogen (H2) has a relative
molecular mass of 2, so 2 grams of
hydrogen (one mole) contains the same
number of molecules as atoms in 12g of
carbon-12 (6.02 x 1023)
YouTube - Happy Mole Day
to You Chemistry Song
YouTube - Junior Chemistry:
The Mole 1
YouTube - The Mole Amadeo Avogadro's Number
Moles!
It follows therefore that 7g of lithium
(atomic mass 7), 20g neon (atomic mass 20)
or 39 g potassium (atomic mass 39) all
contain the same number of atoms (1 mole
or 6.02 x 1023 atoms)
Moles!
• The number of moles of a substance can
thus be found by dividing the mass of
substance by its relative atomic or
molecular mass
n = mass/RAM
Example
How many moles of sulphur atoms are
there in 80g of sulphur? How many grams
of carbon would have the same number of
atoms?
Example
How many moles of sulphur atoms are
there in 80g of sulphur? How many grams
of carbon would have the same number of
atoms?
N = mass/RAM = 80/32 = 2.5 moles
Example
How many moles of sulphur atoms are
there in 80g of sulphur? How many grams
of carbon would have the same number of
atoms?
N = mass/RAM = 80/32 = 2.5 moles
Mass of carbon = RAM x n = 12 x 2.5 = 30 g
Relative formula mass
We can use the idea of moles and apply it to
molecules using relative formula mass.
C2H5OH
RFM = (2 x 12) + (6 x 1) + (1 x 16) = 46
46g of ethanol = I mole of ethanol molecules
Let’s try some questions!
Page 81
Questions 1 to 8
Can you read
through the
“Moles and
gases” section
page 96 and 97?
Moles of gases
• YouTube - Happy Mole Day to You
Chemistry Song
• YouTube - Junior Chemistry: The Mole 1
• YouTube - The Mole - Amadeo Avogadro's
Number
Equal volumes
Luckily, equal volumes of gas contain the
same number of particles (at the same
temperature and pressure)
Equal volumes
For example, 1 litre of nitrogen contains the
same number of molecules as there are
atoms in one litre of Argon.
A litre is 1000 cm3 or 1 dm3
Mole of gas
One mole of any gas occupies 24 dm3
(24000 cm3) at room temperature and
pressure (25°C and 1 atmosphere)
You will be
given this in a
question
At r.t.p. (25°C and 1 atmosphere)
Moles of gas = volume (cm3)/24000 = volume (dm3)/24
Learn
this!
Standard temperature (0°C) and
pressure (s.t.p.)
More questions!
Moles in solution
Concentration/molarity
1M or 1 mol/dm3 means there is one mole
of a substance dissolved in 1 dm3 (or 1000
cm3) of solution
# of moles = concentration x volume (cm3)/1000
You need to know
this too!
Let’s do some more
reading and then try
some more questions!
(pages 344 to 347)
Working out the formula
Working out the formula
• 1.4 g of Nitrogen reacts with 0.3 g of
hydrogen to form a compound. What is the
formula of the compound?
Working out the formula
• 1.4 g of Nitrogen reacts with 0.3 g of
hydrogen to form a compound. What is the
formula of the compound?
• First work out the number of moles
• 1.4g Nitrogen = mass/RAM = 1.4/14 = 0.1
• 0.3g hydrogen = mass/RAM = 0.3/1 = 0.3
Working out the formula
• 1.4 g of Nitrogen reacts with 0.3 g of
hydrogen to form a compound. What is the
formula of the compound?
• Work out the ratio of the number of moles
of each element to the lowest whole
numbers
N
:
H
0.1 :
0.3
1
:
3
Working out the formula
• 1.4 g of Nitrogen reacts with 0.3 g of
hydrogen to form a compound. What is the
formula of the compound?
• Therefore there are 3 times as many
hydrogen atoms as N atoms in the
compound. The formula must be NH3
(ammonia)
Working out the formula
• A student adds 4.8 g of magnesium to
excess dilute hydrochloric acid. What mass
of magnesium chloride would be made?
Working out the formula
• A student adds 4.8 g of magnesium to
excess dilute hydrochloric acid. What mass
of magnesium chloride would be made?
• First write the balanced equation
Mg + 2HCl
MgCl2 + H2
Working out the formula
• A student adds 4.8 g of magnesium to
excess dilute hydrochloric acid. What mass
of magnesium chloride would be made?
• Find out how many moles of magnesium
are being used
# moles = mass/RAM = 4.8/24 = 0.2 moles
Working out the formula
• A student adds 4.8 g of magnesium to excess
dilute hydrochloric acid. What mass of
magnesium chloride would be made?
• From the equation we can see the number of
magnesium atoms is the same as the number of
magnesium chloride “molecules”. So if 0.2 moles
of Mg are used we should produce 0.2 moles of
MgCl2
Mg + 2HCl
MgCl2 + H2
Working out the formula
• A student adds 4.8 g of magnesium to
excess dilute hydrochloric acid. What mass
of magnesium chloride would be made?
• Work out the mass of 0.2 moles of MgCl2
RFM (RMM) = 24 + (2 x 35.5) = 95
Mass of 0.2 moles of MgCl = 0.2 x 95 =
19g
Let’s try some more questions!
Pages 82 and 83
Read through the
sheet first!
Norwegian just
discovering the cost
of a beer in Oslo
I feel a test coming on moles!
Thursday 19th June