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 A small
number just talks about the element it
is behind. E.g. O2 = 2 x O
 Small numbers outside the bracket mean
multiply everything inside the bracket by
that number E.g. (CO3)3 = 3 x C, 9 x O
 Where
do we find this information?
mass
number
40
20
 The
Ca
mass number is the top number in the
periodic table
Relative Atomic Mass (Ar) tells us the mass of
an atom compared to 1 atom of carbon.
Carbon is given an Ar of 12 (as it has 6 protons
and 6 neutrons).
e.g. Ar of magnesium =
Ar of fluorine =
Ar of calcium =
 The
relative formula mass (Mr) of a
compound is the mass of all the elements in
the compound added together.
 EXAMPLE:
Water
Mass of oxygen = 16
O
H
H
Mass of hydrogen = 1
So the Mr for water = 16 + (2x1) = 18
We can use relative atomic masses to work out
the relative formula mass (Mr)/relative
molecular mass of a molecule or compound.
E.g. NaCl
Ar of Na = 23
Ar of Cl = 35.5
So Mr of NaCl = 23 + 35.5 = 58.5
E.g.2 H2O
Ar of H = 1
Ar of O = 16
So Mr of H2O = (2 x 1) + 16 = 18
• Calculate the relative molecular mass Mr / relative
formula mass for the following compounds.
• Show your working
HCl
NaOH
CaCO3
H2SO4
Ca(OH)2
1
+ 35.5 = 36.5
23 + 16 + 1 = 40
40 + 12 + (3x16) = 100
(2x1) + 32 + (4x16) = 98
40 + (2x16) + (2x1) = 74
PROGRESS
CHECK
 Mr of
CO2
ADD
 Mr
of NaCl
ADD
 Mr
of MgO
142.5
PROGRESS
CHECK
Mr
of CuSO4
SUBTRACT
Mr
of Al2O3
58
PROGRESS
CHECK
Mr
of H2O
ADD
Mr
of ammonia
35
Calculate the relative formula mass of the following
compounds (showing your working!):
(a) Carbon monoxide CO
(b) Carbon dioxide
(c) Sulphur dioxide SO2
(d) Calcium carbonate CaCO3
(e) Sodium hydroxide NaOH
(f) Sulphuric acid H2SO4
(g) Hydrochloric acid HCl
(h) Copper sulphate CuSO4
(i) Magnesium chloride MgCl2
(j) Sodium carbonate Na2CO3
(k) Lead nitrate Pb(NO3)2
(l) Calcium hydroxide Ca(OH)2
DEMONSTRATE
 How

much gold is in your mobile phone?
Use the procedure to calculate the percentage
composition of gold in a mobile phone
 PERCENTAGE
= MASS OF ELEMENT X 100
Mr OF ENTIRE FORMULA
1. (i)
160
1
(ii)
112
1
(iii)
70
1
do not carry forward errors
[3]
2. (a)
evidence of Fe = 56, O = 16
gains 1 mark
but
(56 × 2) + (16 × 3)
gains 2 marks
but
160
gains 3 marks
(b) evidence of 112*/160
but
70*
* (credit answers consistent with part (a))
gains 1 mark
gains 2 marks
[5]
QUESTION 3
• Mr for strychnine is (12 x21) + (1x22) + (14 x
2) + (16x2) = 334
• The percentage mass of carbon in strychnine
is therefore:
252 x 100 = 75.4%
334
• This is not the same as the percentage mass
of carbon in the white powder – so the white
powder is not strychnine.
Calculating percentage composition of
compounds
Percentage
mass of an
element in a
compound
=
Atomic mass Ar x Number of atoms
x 100
Molecular mass Mr
Q. Find the percentage mass of sodium in sodium carbonate Na2CO3
Ar of sodium =
Ar of carbon =
Mr of Na2CO3 =
Percentage
mass of an
element in a
compound
=
Ar of oxygen =
Find the percentage of oxygen in Fe2O3
Find the percentage of nitrogen in NH4NO3
Theoretical Yield – What you should make



of



We can use masses in a reaction to help us calculate the amount
of reactant and product.
Step 1 - Write out the equation for the reaction. Make sure it is
balanced.
Step 2 - Work out the relative masses of the substances needed
in
the calculation. Remember to multiply by the number
molecules that are present.
Step 3 - Convert the relative masses into the units in the
question.
Step 4 - Find the ratio by dividing both numbers by the smallest
relative mass.
Step 5 - Find the mass of the unknown by multiplying the mass of
the known by the ratio of the unknown.
Percentage Yield
 Percentage
 The
Yield =
Actual Yield
Theoretical Yield
X 100
Actual Yield is how much you have
made from the reaction (from the question).
 The Theoretical Yield is how much you
should make if you have no loss what so ever
(100% efficient).