Chapter 3: Chemical
Formulae and
Equations
3.1 Relative Atomic Mass and
Relative Molecular Mass
Mass of Nitrogen
Relative atomic mass, Ar
The mass of an atom is too small to
measure.
 We can only measure the mass of an
atom by comparing its mass to the
mass of another atom.

Relative atomic mass, Ar
Relative atomic mass, Ar
Relative atomic mass, Ar

What is relative atomic mass???

The mass of an atom when compared
to a standard atom is known as its
relative atomic mass (Ar). It has no
unit.
Relative atomic mass, Ar
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Since hydrogen atom is the lightest atom, chemists
first started comparing masses of other atoms with the
mass of one hydrogen atom.
We can consider the relative atomic mass of hydrogen
to be 1.
We can compare the mass of other atom to the mass
of the hydrogen atom.
An atom of carbon-12 is 12 times heavier than one
atom of hydrogen.
The relative atomic mass, Ar of one carbon-12 atom is
12.
Relative atomic mass, Ar
Relative atomic mass, Ar
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This method, however is not always
convenient. Do you know why?
Hydrogen is a gas. Therefore it is difficult to
measure its mass.
It has a number of isotopes with different
masses. Thus it is difficult to measure
exactly.
Oxygen was then used as a standard to
compare the masses of atoms, however,
this also posed some problems.
Relative atomic mass, Ar
Relative atomic mass, Ar
At present carbon-12 is used as the standard
of comparison because
 It is solid and can be easily handled,
 Its mass can be more easily measured with
a mass spectrometer.
 Carbon-12 is easily found as its compounds
are abundant.
 Carbon-12 isotopes is assigned a mass of
exactly 12 units.

Relative atomic mass, Ar

The relative atomic mass of an
element is the average mass of one
atom of the element when
compared with 1 of the mass of a
12
carbon -12.
Relative atomic mass, Ar
Examples
An atom of oxygen is 16 times heavier
1
than of the mass of an atom of
12
carbon-12. Its relative atomic mass is
16.
 An atom of sodium is 23 times heavier
1
than 12 of the mass of an atom of
carbon-12. Its relative atomic mass is
23.

Relative atomic mass, Ar
Since relative atomic mass compares
the masses of atoms , it does not
have any units.
 The relative atomic mass is also
related to the number of protons and
neutrons in the atom.

The symbols, relative mass and
the charge of subatomic
particles
Subatomic
particle
Symbol
Relative
mass
Charge
Proton
p
1
+1
Electron
e
1
1840
-1
Neutron
n
1
0
Relative atomic mass, Ar

If more than one isotope of element is
present, the atomic mass is calculated
by taking the average of the relative
atomic masses of different isotopes.
Examples of isotopes of some elements
Element
Proton
number
Nucleon
number
Number of Number of
protons
neutrons
Percentage
abundance
Hydrogen, 11 H
Deuterium,21 H
Tritium,31 H
1
1
1
1
2
3
1
1
1
0
1
2
99.985%
0.015%
Man-made
isotope
6
6
6
12
13
14
6
6
6
6
7
8
98.1%
1.1%
0.8 %
17
17
35
37
17
17
18
20
75.5%
24.5%
8
8
8
16
17
18
8
8
8
8
9
10
99.757%
0.038%
0.205%
12
Carbon-12, 6 C
13
Carbon-13, 6 C
Carbon-14, 146 C
35
17
Chlorine-35, Cl
Chlorine-37, 3717 Cl
16
8
Oxygen-16, O
Oxygen-17, 178 O
Oxygen-18, 188 O
Example

How many times heavier is a krypton
atom compared to a helium atom?

Solution:
mass of a krypton atom = Ar of krypton
mass of a helium atom Ar of helium
= 84
4
= 21 times
Example

Calculate how many times heavier are 3
calcium atoms compared to 5 carbon
atoms.

Solution:
mass of 3 calcium atoms = 3 x Ar of calcium
mass of 5 carbon atoms 5 x Ar of carbon
= 3 x 40
5 x 12
= 21 times
Relative Molecular Mass, Mr
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The idea of relative atomic mass can be
extended to molecules.
The Relative molecular mass of a molecule
is the average mass of the molecule when
compared with 1 of the mass of one atom
carbon-12.
12
Example: Relative Molecular Mass of CO2
is 44.
This means that the molecule of carbon
dioxide is 44 times heavier than 1 of one
atom of carbon-12
12
Relative Molecular Mass, Mr
Relative Molecular Mass, Mr
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Since a molecule is made up of atoms, the
relative molecular mass of a molecule can be
calculated by adding up the relative atomic
masses of all the atom present in the
molecule.
Calculation of Relative
Molecular Mass
Step 1: Determine the molecular
formula.
 Step 2: Find the relative atomic mass
of each element in the
molecule.
 Step 3: Add up all the relative atomic
masses of the element or
elements.
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Relative Molecular Mass, Mr
Molecule
Relative molecular mass
Chlorine
Molecular
formula
Cl2
Nitrogen
N2
2 x 14 = 28
Ammonia
NH3
14 + (3 x 1) = 17
Ethanol
C2H5OH
(2 x 12) + (5 x 1) + 16 + 1 = 46
Carbon
dioxide
CO2
12 + (2 x 16) = 44
2 x 35.5 = 72
Relative formula masses
Some substances consists of ions and
not molecules.
 For these ionic substances, the
relative formula masses are used in
place of relative molecular masses.

Relative formula masses
Ionic
substance
Ionic
formula
Relative formula mass
Sodium
chloride
NaCl
23 + 35.5 = 58.5
Hydrated
magnesium
sulphate
Calcium
oxide
MgSO4.7 24 + 32 + (4x 16) + 7(2 +
H2O
16) = 246
CaO
40 + 16 = 56
Example
The relative formula mass of a compound with the
formula Y2SO4 is 142. Calculate the relative atomic
mass of element Y.
 [ Relative atomic mass: O, 16; S,32]
 Solution:
 Let the relative atomic mass of element Y=y. Given
that the relative formula mass of Y2SO4 =142
 Therefore, 2y + 32 + 4(16) = 142
2y + 96 = 142
2y = 142 – 96
2y = 46
y = 23
So, the relative atomic mass of element Y is 23.
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