Additional Chemistry
Calculations
Relative atomic and Formula Masses
The mass of an atom is too small to deal with in real terms, so we use
‘relative’ masses – Carbon is given a mass of 12, and everything else is
compared with it and given a mass, e.g. Oxygen is ‘heavier’, so its
relative mass is 16.
The relative atomic mass can be found by looking at
the periodic table, It is always the larger of the two
numbers.
Relative formula mass can be found by adding up the relative atomic
masses of each element in a compound.
E.g. Carbon Dioxide (CO2)
Carbon has a relative atomic mass of 12
Oxygen has a relative atomic mass of 16
The relative formula mass of Carbon Dioxide is therefore:
12 + (16 x 2) = 44
x 2 because
it’s “O2”
Moles
Because saying ‘relative formula (or atomic!) mass in grams’ is a bit
clumsy, we simply say ‘moles’ instead. This means that 1 mole of Carbon
Dioxide is 44 grams, or 44g. Simple!
Percentage of an element in a compound
We can use the relative atomic mass (Ar) of elements and the relative
formula mass (Mr) of compounds to find out the percentage composition
of different elements.
E.g. What percentage mass of white Magnesium Oxide is actually
Magnesium, and how much is Oxygen?
1.
2.
3.
Work out the mass of MgO
24 + 16 = 40
Convert to grams
40g
1. Work out the formula mass of
the compound
2. Convert this into grams
3. Work out the percentage by
using this equation:
Mass of element
x 100%
Total mass of compound
Work out the percentage
24 X 100% = 60% is Magnesium, so 40% must be Oxygen!
40
Formula of a compound from its percentage composition
We can also do this backwards! If we know the percentage composition
of a compound we can work out the ratio of atoms. This is known as the
Empirical Formula. Sometimes this is the same as the molecular
formula, but not always (e.g. water has an empirical and molecular
formula of H2O. Hydrogen peroxide's empirical formula is HO, but it’s
molecular formula is H2O2.
E.g. If 9g of Aluminium react with 35.5g of Chlorine, what is the
empirical formula of the compound formed?
Aluminium
9
= 1/3 moles of Aluminium atoms
27
Chlorine
35.5
= 1 mole of Chlorine atoms
35.5
1. Divide the mass of each
element by its relative
atomic mass to find out the
number of moles reacted
2. Create a ratio and simplify if
necessary
3. Write a formula based on
the ratio
Al : Cl
1/3 : 1
1 : 3
AlCl3
Masses of reactants and products
This is an important calculation when we want to know how much of each
reactant to react together. For example, sodium hydroxide reacts with
chlorine gas to make bleach. If we have too much Chlorine, some will be
wasted. Too little and not all of the sodium hydroxide will react.
2NaOH + Cl2  NaOCl + NaCl + H2O
Sodium Chlorine Bleach
Hydroxide
Salt
Water
How much Chlorine gas should we bubble through 100g of Sodium
Hydroxide to make Bleach?
1. NaOH
23 + 16 + 1 = 40g is one mole of NaOH
2. We have 100g in our reaction so…
100 = 2.5 moles
40
1. Work out the mass of one
mole of Sodium Hydroxide
2. Calculate how many moles
you have in your reaction
3. Work out how many moles
of Chlorine you need
4. Convert this into a mass
for Chlorine
3. The chemical equation tells us that we need 2 moles of Sodium
Hydroxide (2NaOH) for every mole of Chlorine (Cl2).
So we need: 2.5 = 1.25 moles of Chlorine
2
4. 35.5 x 2 = 71g is one mole of Cl2
So we need 1.25 x 71 = 88.75g of Chlorine to react with 100g of
Sodium Hydroxide.
Percentage Yield
Rather than talk about the yield of a chemical reaction in terms of mass
(grams, tonnes etc.) we can talk about the percentage yield. This gives
us an idea of the amount of product that the reaction really makes,
compared to what it could possibly make under perfect conditions.
There are many reasons why we don’t make 100% every time, such as:
–
–
–
–
The reaction may be reversible
Some product could be left behind in the apparatus
The reactants may not be pure
It may be difficult to separate the products if more than one are made.
Using this reaction “A + B  C”, it was found that in perfect conditions,
scientists could make 2.5g of C. However, when they tried it out, they
only made 1.5. What is the percentage yield of this reaction?
Amount of product produced
x 100%
Maximum amount of product possible
1.5 x 100% = 60% percentage yield
2.5
The higher the
percentage yield and
atom economy, the
better the reactions are
for the Earth’s
resources, as there’s less
waste!