Calculations Revision Sheet
Relative Formula Mass
Identify the relative atomic masses of each element in the compound
Multiply the relative atomic mass by the number of atoms in that
element
Add up the results for each element
E.g: H2O
RAM: H=1, O=16
=(1 x 2)+(1 x 16)
= 18
Moles
o 1 mole = 6.02 x 10*23 (also known as the Avogadro constant)
o For an element, the mass of 1 mole is the same as the RAM
o The RFM of a substance is equal to the mass of 1 mole of a substance (used if
the substance is a compound)
o The equation for finding the moles in a substance =
Moles = mass/RFM
E.g: the Ar of aluminium is 27, how many moles are there in 54g of aluminium?
: Moles = mass/RAM or RFM
: Moles = 54/27
= 2 moles
Percentage Yield
o Percentage yield is the comparison of the real yield of an experiment vs the
theoretical yield of the reaction.
o The Law of the conservation of mass shows that all reactants used must equal
the products made
o However this varies due to side reactions, incomplete reactions, difficulty in
extracting the products and waste.
o Therefore, percentage yield is always below 100%
o It is calculated as:
Percentage Yield = Real yield ÷ theoretical yield x 100
Empirical Formula
o Empirical formula is the simplest ration of atoms which make up a molecule
Step Action Result
Write the element
1 S O
symbols
2 Write the masses 3.2 g 6.4 g – 3.2 g = 3.2 g
3 Write the Ar values 32 16
Relative Formula Mass
Identify the relative atomic masses of each element in the compound
Multiply the relative atomic mass by the number of atoms in that
element
Add up the results for each element
E.g: H2O
RAM: H=1, O=16
=(1 x 2)+(1 x 16)
= 18
Moles
o 1 mole = 6.02 x 10*23 (also known as the Avogadro constant)
o For an element, the mass of 1 mole is the same as the RAM
o The RFM of a substance is equal to the mass of 1 mole of a substance (used if
the substance is a compound)
o The equation for finding the moles in a substance =
Moles = mass/RFM
E.g: the Ar of aluminium is 27, how many moles are there in 54g of aluminium?
: Moles = mass/RAM or RFM
: Moles = 54/27
= 2 moles
Percentage Yield
o Percentage yield is the comparison of the real yield of an experiment vs the
theoretical yield of the reaction.
o The Law of the conservation of mass shows that all reactants used must equal
the products made
o However this varies due to side reactions, incomplete reactions, difficulty in
extracting the products and waste.
o Therefore, percentage yield is always below 100%
o It is calculated as:
Percentage Yield = Real yield ÷ theoretical yield x 100
Empirical Formula
o Empirical formula is the simplest ration of atoms which make up a molecule
Step Action Result
Write the element
1 S O
symbols
2 Write the masses 3.2 g 6.4 g – 3.2 g = 3.2 g
3 Write the Ar values 32 16
