Formulae masses - using the Periodic Table
All chemical compounds have a chemical formula
If the substance contains only:
 single atoms - we talk about ATOMIC FORMULA e.g. helium He, neon Ne.
 molecules - we talk about MOLECULAR FORMULA e.g. iodine I2 , methane CH4
 ions - we talk about IONIC FORMULA e.g. sodium chloride Na+ Cl Why RELATIVE atomic masses (R.A.M) ?
If we weigh atoms of various elements we find that the individual masses are very small.
Name
Mass of 1 atom
Mass of 1 atom in
Approximate Relative
atomic mass units
atomic mass
Hydrogen
1.0078 x 1.66 x 10-27 kg
1.0078
1
Helium
4.0026 x 1.66 x 10-27 kg
4.0026
4
Lithium
7.016 x 1.66 x 10-27 kg
7.016
7
Carbon
12.000 x 1.66 x 10-27 kg
12.000
12
Neon
19.992 x 1.66 x 10-27 kg
19.992
20
NOTE
An atomic mass unit (amu) is defined as 1/12 the mass of the carbon-12 isotope.
So carbon-12 is defined as weighing exactly 12 amu. This is the starting point for how much an
atom weighs.
So if you weigh 1/2 as much as the carbon-12 atom, you weigh 6 amu.
If you weigh twice as much as the carbon-12 atom, you weigh 24 amu.
CALCULATING FORMULA MASSES
We need a periodic table for looking up atomic masses and a calculator.
Look at the following flow chart - it gives us some idea what needs to be done.
Chemical formula
H2O
Elements present
Hydrogen
Oxygen
Relative atomic mass of each
element
Hydrogen = 1
Oxygen = 16
Number of atoms of each
element
Hydrogen = 2
Oxygen = 1
Total mass of each element
Hydrogen =2 x 1 = 2
Oxygen = 1 x 16 = 16
Total mass of compound
Water = H2O = 2+16
Water = 18
NOTE 1
You need to know how many atoms of each element are in a substance in order to calculate its
formula mass.
For example
Water (H2O) has two atoms of hydrogen and one atom of oxygen.
Hydrogen peroxide (H2O2) has two atoms each of oxygen and hydrogen.
Magnesium hydroxide (Mg(OH) 2) has one atom of magnesium and two each of oxygen and
hydrogen.
PROBLEMS: WHAT'S IN A COMPOUND ?
Name the element and work out the number of atoms of each element present in the following
compounds.
Example 1: Potassium chloride KCl
Formula
KCl
Element
potassium = 1
No of atoms
Chlorine = 1
Example 2: Iron (III) oxide Fe2O3
Formula
Fe2O3
Element
iron
oxygen
No of atoms
2
3
Example 3: Aluminium (III) nitrate Al (NO3)3
For brackets e.g (NO3) 3 this is the same as 3 x NO3 = 3N and 3 x N3 = 9N
Formula
Al(NO3)3
Element
aluminium
nitrogen
oxygen
No of atoms
1
1 x 3 =3
3x3=9
NOTE 2
You need to know the atomic mass of each element in order to calculate the molecular mass of the
compound.
The atomic mass of each element is obtained from the periodic table.
Just below are typical entries in theperiodic table for hydrogen and oxygen.
HOW TO CALCULATE THE MOLECULAR MASS OF A SUBSTANCE
Example 1 - Hydrogen peroxide H2O2 - follow the example below
Formula
Element
No of atoms
Atomic Mass
of element
H2O2
Hydrogen
Oxygen
2
2
1
16
Total mass of
atoms in
element
2x1=2
2 x 16 = 32
TOTAL
MASS of the
compound
2 +32
= 34
TOTAL
MASS of the
compound
24 +32 + 2
= 58
Hence mass of H2O2 = 34
Example 2 - Magnesium hydroxide Mg(OH) 2
Formula
Element
No of atoms
Atomic Mass
of element
Mg(OH) 2
Magnesium
1
24
Total mass of
atoms in
element
1 x 24 = 24
Oxygen
Hydrogen
2
2
16
1
2 x 16 = 32
2x1=2
Atomic Mass
of element
Hence mass of Mg(OH) 2 = 58
Example 3 - Aluminium sulphate Al2 (SO4) 3
Formula
Element
No of atoms
Al2 (SO4) 3
Aluminium
2
27
Total mass of
atoms in
element
2 x 27 = 54
Sulphur
Oxygen
3
12
32
16
3 x 32 = 96
12 x 16 = 192
TOTAL
MASS of the
compound
54 +96 + 192
= 342
Hence mass of Al2 (SO4) 3 = 342
Example 4 - Copper (II) sulphate pentahydrate CuSO4.5H2O:
Treat this as two compounds
Copper (II) sulphate: Cu SO4 and pentahydrate: 5H2O
Formula
Element
No of atoms
Atomic Mass
of element
CuSO4
Copper
1
64
Total mass of
atoms in
element
1 x 64 = 64
Sulphur
Oxygen
Hydrogen
Oxygen
1
4
10
5
32
16
1
16
1 x 32 = 32
4 x 16 = 64
10 x 1= 10
5 x 16 = 80
5H2O
CuSO4.5H2O
Hence mass of CuSO4.5H2O = 250
TOTAL
MASS of the
compound
64 +32 + 64
= 160
10 + 80
= 90
160 + 90 =
250