Chemical calculations
Year 11
Starter
• Write down all the
information you can
about the atoms of the
element shown right
Learning objectives
By the end of the lessons you should be able to:
• calculate relative formula mass from relative
atomic masses
• use chemical equations to calculate masses of
reactants and products.
Key words
• relative atomic mass, relative formula mass
Masses of atoms
• Why don’t we measure the
masses of atoms in grams
and milligrams?
• They are too small
• How do we measure the
mass of atoms?
• What subatomic particles
give an atom mass?
• What do you think the mass
number tells us?
• The number of protons and
neutrons
• What would the atomic
number of carbon be?
Masses of molecules
• We use Carbon-12 as a
standard atom, with a
mass of exactly 12 units
• Relative formula mass
(Mr) tells us how heavy
a molecule is compared
to a carbon-12 atom
• How could you calculate
the Mr of water?
Example 2
• What is the Mr of
sodium hydroxide?
Example 3
• What is the Mr of
calcium hydroxide?
Task one
• Complete the worksheet C2.5 8b Relative
atomic and formula masses
Limestone
• Very useful rock
• Can be used to make
calcium oxide which is used
to neutralise acidic soil
• Heating calcium carbonate
causes it to thermally
decompose into calcium
oxide and carbon dioxide
• CaCO3(s)  CaO(s) + CO2(g)
How much?
• How could we work it out?
• By using the RFM
• What is the RFM of calcium
carbonate?
• And calcium oxide?
• CaCO3(s)  CaO(s) + CO2(g)
100
56
44
• This means that the ration of
calcium carbonate to calcium
oxide is 100:56
• How much calcium oxide will we
make?
Burning magnesium
• What does magnesium form
when it burns in air?
• Magnesium oxide
• If we burned 1.2g of
magnesium how much
magnesium oxide would we
make?
• 2Mg(s) + O2(g)  2MgO(s)
• Mg = 24
• O = 16
Task two
• Complete the worksheet C6.6a Calculating
amounts
Task one