% composition and molecular formulae
Simple ideas
Imagine you have a boy and a girl in a class.
The % of boys in the class = number of boys / total in class
The % of boys in the class
= 1/2 x 100 = 50%
In this example we have 6 green and 3 orange balls
% of green balls = number of green balls / total number of balls x 100
% of green balls = 6/9 x 100 = 66.7%
% of orange balls = number of orange balls / total number of balls x 100
% of orange balls = 3/9 x 100 = 33.3%
CHECK that the total % = 66.7% + 33.3% = 100%
In this example we have 8 blue, 12 yellowish green and 5 orange balls.
Total number of balls = 8 + 12+ 5 = 25
% of blue balls = 8 / 25 x 100 = 32%
% of yellowish green balls = 12/25 x 100 = 48%
% of orange balls = 5 / 25 x 100 = 20%
CHECK that the total % = 32% + 48% + 20% =
100%
SUMMARY
We can find the % composition (by mass) of a substance if we know
 the chemical formula of the substance
 the mass of the substance
 the amount and mass of each of the elements making up the substance.
EXAMPLE 1 Find the % composition of the elements present in water H2O.
The chemical formula of the substance =H2O.
The mass of the substance = 2 x 1 + 16 = 18g
The mass of oxygen = 16g - hence % of oxygen in water = 16/18 x 100 = 88.9%
The mass of hydrogen = 2g - hence % of hydrogen in water = 2/18 x 100 = 11.1 %
If we put this in the form of a table:
Elements
Atomic Mass
number of
mass of element
% composition of the element
present
of element (g)
atoms present
in compound (g)
present in the compound
Hydrogen
1
2
2x1=2
2 / 16 x 100 = 11.1%
Oxygen
16
1
1 x 16 = 16
16 / 16 x 100 = 88.9%
TOTAL MASS
is 16 + 2 = 18g
TOTAL % is
11.1+ 88.9 = 100%
EXAMPLE 2 : Find the % composition of the elements present in ethanol C2H5OH.
If we put this in the form of a table:
Elements
Atomic Mass
number of
mass of element in
% composition of the
present
of element (g)
atoms
compound (g)
element present in the
present
compound
Hydrogen
1
6
6x1=6
6 / 46 x 100 =13.0 %
Oxygen
16
1
1 x 16 = 16
16 / 46 x 100 =34.8 %
Carbon
12
2
2 x 12 = 24
24 / 46 x 100 = 52.2%
TOTAL MASS is
% TOTAL =
6+16 + 24 = 46g
13+ 34.8+ 52.2 = 100%
Exercise on % composition
ChR3h1.htm
Percentage Formulae (1)
Work through the following exercise checking how you get on.
1.Find the % composition of the elements present in methanol CH3OH.
(Atomic masses C = 12, H = 1, O = 16).
Compound
Atomic
No. of atoms
Mass of
Total mass of
% composition of each
mass of
of element
element in
compound
element in the compound
element
present
compound
C = 12
C=
C=
CH3OH
H=1
0 = 16
2.Find the % composition of the elements present in ethanoic acid CH3 COOH.
(Atomic masses C = 12, H = 1, O = 16).
Compound
Atomic
No. of atoms
Mass of
Total mass of
% composition of each
mass of
of element
element in
compound
element in the compound
element
present
compound
C = 12
C=
C=
C=
CH3 COOH
H=1
H=
0 = 16
O=
3. % composition of various fertilisers
Which of the fertilisers below contains the greatest percentage of nitrogen
(Atomic masses C = 12, H = 1, O = 16, N = 14).
Fertiliser
Formula
Atomic
mass of
element
Ammonia
NH3
N = 14
Urea
Ammonium
Nitrate
No. of
atoms of
nitrogen
present
Mass of
Total mass
nitrogen in
of
compound compound
%
composition
of nitrogen in
the compound
N=
%
N=
%
N=
%
CO(NH2)2
NH4NO3
Hence the fertiliser with the most nitrogen in it is ......................................................................