Molecular Formula
A molecular formula shows the elements that a compound is made out of and the
number of atoms of each element present in one molecule of the compound. The
numbers are shown written as subscripts.
e.g.
The molecular formula of butane is C4H10
One molecule of ethane contains 4 carbon atoms and 10 hydrogen atoms.
The molecular formula of water is H2O
One molecule of water contains 2 hydrogen atoms and one oxygen atom.
Empirical Formula
The empirical formula gives the simplest whole number ratio of each type of
atom in a compound
e.g.
The molecular formula of butane is C4H10
The empirical formula of butane is C2H5
Relative molecular mass
Molecules are incredibly light so when discussing how heavy a molecule is Chemists often
find the relative molecular mass. This is measured in atomic mass units. The table below
shows the relative atomic masses of various elements which can be used to calculate the
relative molecular mass of molecules.
e.g.
The molecular formula of butane is C4H10
The relative molecular mass of ethane is 4 x 12 + 10 x 1 = 58 atomic mass units
Element
Hydrogen
Carbon
Oxygen
Nitrogen
Calcium
Silicon
Symbol
H
C
O
N
Ca
Si
Relative Atomic Mass
1
12
16
14
40
28
Elemental Analysis
Is a technique that allows a chemist to workout (as a percentage) how much each
element contributes to the total mass of the molecule. If you know these percentages
you can work out what the empirical formula of a molecule so it can help Chemists work
out what they have made.
Ratio
Percentages
Fractions
Substitution
Use your skills with fractions, percentages, ratios and
substitution to work out missing information about
molecules and even identify mystery compounds.
Use the table below to help you.
Element
Hydrogen
Carbon
Oxygen
Nitrogen
Calcium
Silicon
Symbol
H
C
O
N
Ca
Si
Relative Atomic Mass
1
12
16
14
40
28
Molecular
Formula
C3H6
C12H18
C6H12O6
C6H6
CaCO3
SiH4
Ratio
3:6
Empirical
formula
CH2
Simplified
Ratio
1:2
Ratio of
masses
12:2
Fraction of
total mass for
each element
12 2
:
14 14
% by mass
C: 85.7%
H: 14.3%
Relative
molecular mass
(amu)
42
There a few different ways of using this resource:
1. Fill in the gaps in the table (possibly in pairs- each
use a different colour)
2. Cut up the completed grid and use it as a card
sort (possibly only use one slide rather than two)
3. Print the reversed complete grid with a picture on
the back of the page before you cut it out into
cards. Put the cards in the correct places on the
template grid. Flip over each card to reveal the
picture (and hence check the cards are in the
correct place.
• Fill in gaps (given formula)
Molecular
Formula
C3H6
C12H18
C6H12O6
C6H6
CaCO3
SiH4
Ratio
Empirical
formula
Simplified
Ratio
Ratio of
masses
Fraction of
total mass for
each element
% by mass
Relative
molecular mass
(amu)
Molecular
Formula
Ratio
Empirical
formula
Simplified
Ratio
Ratio of
masses
Fraction of
total mass for
each element
% by mass
Relative
molecular mass
(amu)
CH4N
60
CH4
16
CH3O
62
CH3
30
C5H4
128
CHO2
90
• answers
Molecular
Formula
C3H6
C12H18
C6H12O6
C6H6
CaCO3
SiH4
Ratio
3:6
12:18
6:12:6
6:6
1:1:3
1:4
Empirical
formula
CH2
C2H3
CH2O
CH
CaCO3
SiH4
Simplified
Ratio
1:2
2:3
1:2:1
1:1
1:1:3
1:4
Ratio of
masses
Fraction of
total mass for
each element
12:2
12 2
:
14 14
24:3
24 3
:
27 27
12:2:16
12:1
40:12:48
28:4
12 2 16
: :
30 30 30
12 1
:
13 13
40 12 48
: :
100 16 100
28 4
:
32 32
% by mass
Relative
molecular mass
(amu)
C: 85.7%
H: 14.3%
42
C: 88.9%
H: 11.1%
162
C:40%
H 6.7%
O: 53.3%
C: 92.3%
H: 7.7%
Ca: 40%
C: 12%
O: 48%
Si: 87.5%
H: 12.5%
180
78
100
32
Molecular
Formula
C2H8N2
CH4
C2H6O2
C2H6
C10H8
C2H2O4
Ratio
2:6:2
1:4
2:6:2
2:6
10:8
2:2:4
Empirical
formula
CH4N
CH4
CH3O
CH3
C5H4
CHO2
Simplified
Ratio
1:4:1
1:4
1:3:1
1:3
5:4
1:1:2
Ratio of
masses
12:4:14
12:4
Fraction of
total mass for
each element
12 4 14
: :
30 30 30
12 4
:
16 16
% by mass
C:40%
H: 13.3%
N: 46.7%
C: 75%
H: 25%
Relative
molecular mass
(amu)
60
16
12:3:16
12 3 16
: :
31 31 31
C: 38.7%
H: 9.7%
O: 51.6%
12:3
12 3
:
15 15
C: 80%
H: 20%
30
60:4
60 4
:
64 64
C: 93.75%
H: 6.25%
128
12:1:32
12 1 32
: :
45 45 45
C: 26.7%
H: 2.2%
O: 71.1%
62
90
• Fill in gaps (harder)
Molecular
Formula
Ratio
Empirical
formula
Simplified
Ratio
Ratio of
masses
Fraction of
total mass for
each element
% by mass
Relative
molecular mass
(amu)
C3H6
C12H18
CH2O
180
C:H
78
12 1
:
13 13
Ca: 40%
C: 12%
O: 48%
SiH4
100
32
Molecular
Formula
Ratio
Empirical
formula
Simplified
Ratio
Ratio of
masses
Fraction of
total mass for
each element
% by mass
Relative
molecular mass
(amu)
C:H:N
12 4 14
: :
30 30 30
60
C: 75%
H: 25%
C:H:O
12:3:16
16
62
C: 80%
H: 20%
30
C: 93.75%
H: 6.25%
128
C: 26.7%
H: 2.2%
O: 71.1%
90
• answers
Molecular
Formula
C3H6
C12H18
C6H12O6
C6H6
CaCO3
SiH4
Ratio
3:6
12:18
6:12:6
6:6
1:1:3
1:4
Empirical
formula
CH2
C2H3
CH2O
CH
CaCO3
SiH4
Simplified
Ratio
1:2
2:3
1:2:1
1:1
1:1:3
1:4
Ratio of
masses
12:2
6:1
24:3
8:1
12:2:16
6:1:8
12:1
40:12:48
10:3:12
28:4
7:1
Fraction of
total mass for
each element
12 2
:
14 14
6 1
:
7 7
24 3
:
27 27
8 1
:
9 9
12 2 16
: :
30 30 30
6 1 8
: :
15 15 15
12 1
:
13 13
40 12 48
: :
100 16 100
10 3 12
:
:
25 25 25
28 4
:
32 32
7 1
:
8 8
% by mass
Relative
molecular mass
(amu)
C: 85.7%
H: 14.3%
42
C: 88.9%
H: 11.1%
162
C:40%
H 6.7%
O: 53.3%
C: 92.3%
H: 7.7%
Ca: 40%
C: 12%
O: 48%
Si: 87.5%
H: 12.5%
180
78
100
32
Molecular
Formula
C2H8N2
CH4
C2H6O2
C2H6
C10H8
C2H2O4
Ratio
2:6:2
1:4
2:6:2
2:6
10:8
2:2:4
Empirical
formula
CH4N
CH4
CH3O
CH3
C5H4
CHO2
Simplified
Ratio
1:4:1
1:4
1:3:1
1:3
5:4
1:1:2
Ratio of
masses
12:4:14
6:2:7
12:4
3:1
12:3:16
12:3
4:1
60:4
15:1
12:1:32
Fraction of
total mass for
each element
% by mass
12 4 14
: :
30 30 30
C:40%
H: 13.3%
N: 46.7%
6 2 7
: :
15 15 15
12 4
:
16 16
3 1
:
4 4
C: 75%
H: 25%
Relative
molecular mass
(amu)
60
16
12 3 16
: :
31 31 31
C: 38.7%
H: 9.7%
O: 51.6%
12 3
:
15 15
C: 80%
H: 20%
30
4 1
:
5 5
60 4
:
64 64
15 1
:
16 16
C: 93.75%
H: 6.25%
128
12 1 32
: :
45 45 45
C: 26.7%
H: 2.2%
O: 71.1%
62
90
• Card sort
Molecular
Formula
C3H6
C12H18
C6H12O6
C6H6
CaCO3
SiH4
Ratio
3:6
12:18
6:12:6
6:6
1:1:3
1:4
Empirical
formula
CH2
C2H3
CH2O
CH
CaCO3
SiH4
Simplified
Ratio
1:2
2:3
1:2:1
1:1
1:1:3
1:4
Ratio of
masses
12:2
6:1
24:3
8:1
12:2:16
6:1:8
12:1
40:12:48
10:3:12
28:4
7:1
Fraction of
total mass for
each element
12 2
:
14 14
6 1
:
7 7
24 3
:
27 27
8 1
:
9 9
12 2 16
: :
30 30 30
6 1 8
: :
15 15 15
12 1
:
13 13
40 12 48
: :
100 16 100
10 3 12
:
:
25 25 25
28 4
:
32 32
7 1
:
8 8
% by mass
Relative
molecular mass
(amu)
C: 85.7%
H: 14.3%
42
C: 88.9%
H: 11.1%
162
C:40%
H 6.7%
O: 53.3%
C: 92.3%
H: 7.7%
Ca: 40%
C: 12%
O: 48%
Si: 87.5%
H: 12.5%
180
78
100
32
Molecular
Formula
C2H8N2
CH4
C2H6O2
C2H6
C10H8
C2H2O4
Ratio
2:6:2
1:4
2:6:2
2:6
10:8
2:2:4
Empirical
formula
CH4N
CH4
CH3O
CH3
C5H4
CHO2
Simplified
Ratio
1:4:1
1:4
1:3:1
1:3
5:4
1:1:2
Ratio of
masses
12:4:14
6:2:7
12:4
3:1
12:3:16
12:3
4:1
60:4
15:1
12:1:32
Fraction of
total mass for
each element
% by mass
12 4 14
: :
30 30 30
C:40%
H: 13.3%
N: 46.7%
6 2 7
: :
15 15 15
12 4
:
16 16
3 1
:
4 4
C: 75%
H: 25%
Relative
molecular mass
(amu)
60
16
12 3 16
: :
31 31 31
C: 38.7%
H: 9.7%
O: 51.6%
12 3
:
15 15
C: 80%
H: 20%
30
4 1
:
5 5
60 4
:
64 64
15 1
:
16 16
C: 93.75%
H: 6.25%
128
12 1 32
: :
45 45 45
C: 26.7%
H: 2.2%
O: 71.1%
62
90
• Card sort with template (given formula)
Molecular
Formula
C3H6
C12H18
C6H12O6
C6H6
CaCO3
SiH4
Ratio
Empirical
formula
Simplified
Ratio
Ratio of
masses
Fraction of
total mass for
each element
% by mass
Relative
molecular mass
(amu)
Molecular
Formula
C2H8N2
CH4
C2H6O2
C2H6
C10H8
C2H2O4
Ratio
Empirical
formula
Simplified
Ratio
Ratio of
masses
Fraction of
total mass for
each element
% by mass
Relative
molecular mass
(amu)
Relative
molecular mass
(amu)
% by mass
Fraction of
total mass for
each element
42
C: 85.7%
H: 14.3%
12 2
:
14 14
162
C: 88.9%
H: 11.1%
180
78
100
32
C:40%
H 6.7%
O: 53.3%
C: 92.3%
H: 7.7%
6 1
:
7 7
24 3
:
27 27
8 1
:
9 9
12 2 16
: :
30 30 30
6 1 8
: :
15 15 15
12 1
:
13 13
Ca: 40%
C: 12%
O: 48%
40 12 48
: :
100 16 100
Si: 87.5%
H: 12.5%
28 4
:
32 32
10 3 12
:
:
25 25 25
7 1
:
8 8
Ratio of
masses
Simplified
Ratio
Empirical
formula
Ratio
Molecular
Formula
12:2
6:1
1:2
CH2
3:6
C3H6
24:3
8:1
2:3
C2H3
12:18
C12H18
12:2:16
6:1:8
1:2:1
CH2O
6:12:6
C6H12O6
12:1
1:1
CH
6:6
C6H6
40:12:48
10:3:12
1:1:3
CaCO3
1:1:3
CaCO3
28:4
7:1
1:4
SiH4
1:4
SiH4
(insert picture
of your own choice)
Relative
molecular mass
(amu)
60
16
62
30
128
90
% by mass
Fraction of
total mass for
each element
C:40%
H: 13.3%
N: 46.7%
12 4 14
: :
30 30 30
C: 75%
H: 25%
6 2 7
: :
15 15 15
12 4
:
16 16
3 1
:
4 4
C: 38.7%
H: 9.7%
O: 51.6%
12 3 16
: :
31 31 31
C: 80%
H: 20%
12 3
:
15 15
C: 93.75%
H: 6.25%
4 1
:
5 5
60 4
:
64 64
15 1
:
16 16
C: 26.7%
H: 2.2%
O: 71.1%
12 1 32
: :
45 45 45
Ratio of
masses
Simplified
Ratio
Empirical
formula
Ratio
Molecular
Formula
12:4:14
6:2:7
1:4:1
CH4N
2:6:2
C2H8N2
12:4
3:1
1:4
CH4
1:4
CH4
12:3:16
1:3:1
CH3O
2:6:2
C2H6O2
12:3
4:1
1:3
CH3
2:6
C2H6
60:4
15:1
5:4
C5H4
10:8
C10H8
12:1:32
1:1:2
CHO2
2:2:4
C2H2O4
(insert picture
of your own choice)
• Card sort with template (harder)
Molecular
Formula
Ratio
Empirical
formula
Simplified
Ratio
Ratio of
masses
Fraction of
total mass for
each element
% by mass
Relative
molecular mass
(amu)
C3H6
C12H18
CH2O
180
C:H
78
12 1
:
13 13
Ca: 40%
C: 12%
O: 48%
SiH4
100
32
Molecular
Formula
Ratio
Empirical
formula
Simplified
Ratio
Ratio of
masses
Fraction of
total mass for
each element
% by mass
Relative
molecular mass
(amu)
C:H:N
12 4 14
: :
30 30 30
60
C: 75%
H: 25%
C:H:O
12:3:16
16
62
C: 80%
H: 20%
30
C: 93.75%
H: 6.25%
128
C: 26.7%
H: 2.2%
O: 71.1%
90
Molecular
Formula
C3H6
C12H18
C6H12O6
C6H6
CaCO3
SiH4
Ratio
3:6
12:18
6:12:6
6:6
1:1:3
1:4
Empirical
formula
CH2
C2H3
CH2O
CH
CaCO3
SiH4
Simplified
Ratio
1:2
2:3
1:2:1
1:1
1:1:3
1:4
Ratio of
masses
12:2
6:1
24:3
8:1
12:2:16
6:1:8
12:1
40:12:48
10:3:12
28:4
7:1
Fraction of
total mass for
each element
12 2
:
14 14
6 1
:
7 7
24 3
:
27 27
8 1
:
9 9
12 2 16
: :
30 30 30
6 1 8
: :
15 15 15
12 1
:
13 13
40 12 48
: :
100 16 100
10 3 12
:
:
25 25 25
28 4
:
32 32
7 1
:
8 8
% by mass
Relative
molecular mass
(amu)
C: 85.7%
H: 14.3%
42
C: 88.9%
H: 11.1%
162
C:40%
H 6.7%
O: 53.3%
C: 92.3%
H: 7.7%
Ca: 40%
C: 12%
O: 48%
Si: 87.5%
H: 12.5%
180
78
100
32
Molecular
Formula
C2H8N2
CH4
C2H6O2
C2H6
C10H8
C2H2O4
Ratio
2:6:2
1:4
2:6:2
2:6
10:8
2:2:4
Empirical
formula
CH4N
CH4
CH3O
CH3
C5H4
CHO2
Simplified
Ratio
1:4:1
1:4
1:3:1
1:3
5:4
1:1:2
Ratio of
masses
12:4:14
6:2:7
12:4
3:1
12:3:16
12:3
4:1
60:4
15:1
12:1:32
Fraction of
total mass for
each element
% by mass
12 4 14
: :
30 30 30
C:40%
H: 13.3%
N: 46.7%
6 2 7
: :
15 15 15
12 4
:
16 16
3 1
:
4 4
C: 75%
H: 25%
Relative
molecular mass
(amu)
60
16
12 3 16
: :
31 31 31
C: 38.7%
H: 9.7%
O: 51.6%
12 3
:
15 15
C: 80%
H: 20%
30
4 1
:
5 5
60 4
:
64 64
15 1
:
16 16
C: 93.75%
H: 6.25%
128
12 1 32
: :
45 45 45
C: 26.7%
H: 2.2%
O: 71.1%
62
90
Relative
molecular mass
(amu)
% by mass
Fraction of
total mass for
each element
42
C: 85.7%
H: 14.3%
12 2
:
14 14
162
C: 88.9%
H: 11.1%
180
78
100
32
C:40%
H 6.7%
O: 53.3%
C: 92.3%
H: 7.7%
6 1
:
7 7
24 3
:
27 27
8 1
:
9 9
12 2 16
: :
30 30 30
6 1 8
: :
15 15 15
12 1
:
13 13
Ca: 40%
C: 12%
O: 48%
40 12 48
: :
100 16 100
Si: 87.5%
H: 12.5%
28 4
:
32 32
10 3 12
:
:
25 25 25
7 1
:
8 8
Ratio of
masses
Simplified
Ratio
Empirical
formula
Ratio
Molecular
Formula
12:2
6:1
1:2
CH2
3:6
C3H6
24:3
8:1
2:3
C2H3
12:18
C12H18
12:2:16
6:1:8
1:2:1
CH2O
6:12:6
C6H12O6
12:1
1:1
CH
6:6
C6H6
40:12:48
10:3:12
1:1:3
CaCO3
1:1:3
CaCO3
28:4
7:1
1:4
SiH4
1:4
SiH4
(insert picture
of your own choice)
Relative
molecular mass
(amu)
60
16
62
30
128
90
% by mass
Fraction of
total mass for
each element
C:40%
H: 13.3%
N: 46.7%
12 4 14
: :
30 30 30
C: 75%
H: 25%
6 2 7
: :
15 15 15
12 4
:
16 16
3 1
:
4 4
C: 38.7%
H: 9.7%
O: 51.6%
12 3 16
: :
31 31 31
C: 80%
H: 20%
12 3
:
15 15
C: 93.75%
H: 6.25%
4 1
:
5 5
60 4
:
64 64
15 1
:
16 16
C: 26.7%
H: 2.2%
O: 71.1%
12 1 32
: :
45 45 45
Ratio of
masses
Simplified
Ratio
Empirical
formula
Ratio
Molecular
Formula
12:4:14
6:2:7
1:4:1
CH4N
2:6:2
C2H8N2
12:4
3:1
1:4
CH4
1:4
CH4
12:3:16
1:3:1
CH3O
2:6:2
C2H6O2
12:3
4:1
1:3
CH3
2:6
C2H6
60:4
15:1
5:4
C5H4
10:8
C10H8
12:1:32
1:1:2
CHO2
2:2:4
C2H2O4
(insert picture
of your own choice)
Molecular
Formula
C3H6
C12H18
C6H12O6
C6H6
CaCO3
SiH4
Ratio
3:6
12:18
6:12:6
6:6
1:1:3
1:4
Empirical
formula
CH2
C2H3
CH2O
CH
CaCO3
SiH4
Simplified
Ratio
1:2
2:3
1:2:1
1:1
1:1:3
1:4
Ratio of
masses
12:2
6:1
24:3
8:1
12:2:16
6:1:8
12:1
40:12:48
10:3:12
28:4
7:1
Fraction of
total mass for
each element
% by mass
12 2
:
14 14
C: 85.7%
H: 14.3%
42
C: 88.9%
H: 11.1%
162
6 1
:
7 7
24 3
:
27 27
8 1
:
9 9
12 2 16
: :
30 30 30
6 1 8
: :
15 15 15
12 1
:
13 13
40 12 48
: :
100 16 100
10 3 12
: :
25 25 25
28 4
:
32 32
7 1
:
8 8
C:40%
H 6.7%
O: 53.3%
C: 92.3%
H: 7.7%
Ca: 40%
C: 12%
O: 48%
Si: 87.5%
H: 12.5%
Relative
molecular mass
(amu)
180
78
100
32
Molecular
Formula
C2H8N2
CH4
C2H6O2
C2H6
C10H8
C2H2O4
Ratio
2:6:2
1:4
2:6:2
2:6
10:8
2:2:4
Empirical
formula
CH4N
CH4
CH3O
CH3
C5H4
CHO2
Simplified
Ratio
1:4:1
1:4
1:3:1
1:3
5:4
1:1:2
Ratio of
masses
12:4:14
6:2:7
12:4
3:1
12:3:16
12:3
4:1
60:4
15:1
12:1:32
Fraction of
total mass for
each element
% by mass
12 4 14
: :
30 30 30
C:40%
H: 13.3%
N: 46.7%
6 2 7
: :
15 15 15
12 4
:
16 16
3 1
:
4 4
C: 75%
H: 25%
Relative
molecular mass
(amu)
60
16
12 3 16
: :
31 31 31
C: 38.7%
H: 9.7%
O: 51.6%
12 3
:
15 15
C: 80%
H: 20%
30
4 1
:
5 5
60 4
:
64 64
15 1
:
16 16
C: 93.75%
H: 6.25%
128
12 1 32
: :
45 45 45
C: 26.7%
H: 2.2%
O: 71.1%
62
90
Alternative ratio activity
Match the following molecules that contain the same ratio of elements.
CH2O
C2H2
H2O
C4H8
H2O2
C6H12O3
C5H10
C6H12
C2H4O2
C6H12O6
C2H6O
C2H4
C5H8O
C3H6O3
C6H6
The simplest ratio of elements in a compound is called the empirical formula can you
find the empirical formula for each compound?
Extension
• The mass of each of the compound is
shown on the next slide.
• Can you work out the mass of the
following atoms
H (hydrogen)
C (carbon
O (oxygen)
C2H2 26
CH2O 30
H2O 18
C4H8 56
CH4 16
C6H12O3 132
C5H10 70
C6H12 84
C2H4O2 60
C6H12O6 180
C2H6O 46
C2H4 28
C3H6O3 90
C5H8O 84
C6H6 78