This document discusses relative atomic mass and relative molecular mass. It provides examples to calculate these values. The key points are: 1. Relative atomic mass (Ar) is the average mass of a single atom of an element compared to 1/12 the mass of one carbon-12 atom. 2. The relative molecular mass (Mr) of a molecule is the sum of the relative atomic masses of all the atoms in the molecule. 3. Examples are provided to calculate relative atomic masses and relative molecular masses using atomic mass values and molecular formulas. Formulas, atomic masses, and molecular masses are compared to calculate unknown values.

1. Relative Mass
A relative mass of an object is the comparison of the mass of the object to a
standard object.
Relative Atomic Mass
1. The mass of an atom when compared to another is known as the
relative atomic mass (Ar).
2. The relative atomic mass (Ar) of an element is the average mass of one
atom of the element when compared with 1/12 of the mass of an atom
of carbon-12, which taken as 12 units.
3. 1/12 of the mass of an atom of carbon-12 is named as 1 atomic mass
unit (amu).
4. The mass of one carbon atom is 12 amu.
5. 1/12 of the mass of an atom of carbon-12 is named as 1 atomic mass
unit (amu).
6. The mass of one carbon atom is 12 amu.
Example 1
The mass of a sodium atom is 23 times greater than 1/12 of the mass of
carbon-12 atom. What is the relative atomic mass of sodium?
Answer:
23
Example 2:
The mass of element A is twice of the mass of carbon, therefore its relative
atomic mass is __________. (Relative atomic mass of carbon = 12)
Answer:
Relative Atomic Mass of Element A = 2 x 12 = 24

2. Example 3:
An atom of element X is 13 times heavier than one atom of helium. Calculate
the relative atomic mass of X.(Ar: He = 4 )
Answer:
Relative Atomic Mass of X = 13 x 4 = 52
Example 4:
How many times that the mass of 2 bromine atoms are greater than 4 neon
atoms? (Ar: Ne = 20; Br = 80 )
Answer:
2×80 / 4×20=2
The mass of 2 bromine atoms is 2 time greater than the mass of 4 neon
atoms.
Example 5
4 atoms of element L have same mass as 1 tellurium atom. Find the relative
atomic mass of L. (Ar: Te = 128 )
Answer:
4×L=1×128
L=128 / 4=32
The relative atomic mass of L = 32
Relative Molecular Mass
The relative molecular mass (Mr) of an element is the average mass of one
molecule of the element/compound when compared with of the mass of an
atom of carbon-12, which taken as 12 units.

3. The relative molecular mass of a molecule is equal to the sum of the relative
atomic mass of all the atoms in the molecule.
Example
Find the relative molecular mass of carbon dioxide.
[ JAR: C = 12; O = 16 ]
Answer:
The formula of carbon dioxide = CO2
Relative molecular mass of CO2 = 12 + 2 (16) = 44
Example
What is the relative molecular mass of aluminium sulphate [ Al2(SO4)3]?
( Ar: O = 16, S = 32; Al = 27 )
Answer:
Relative molecular mass of Al2(SO4)3 = 2 (27) + 3[ 32 + 4 (16) ] = 342
Question:
Given that the formula of a compound is KXO3 and its relative molecular mass
is 167. Find the relative atomic mass of element X? (Ar: O = 16; K = 39 )
Answer
Let's say the relative atomic mass of element X = m
The relative molecular mass of KXO3 = 39 + m + 3(16) = 167
m = 167 - 39 - 3(16) = 80
relative atomic mass of element X = 80
Question:
The general formula of a hydrocarbon is CnH2n and the relative molecular mass
of the hydrocarbon is 84. Find the value of n.
Answer:

4. Relative molecular mass of CnH2n = n(12) + 2n(1) = 84
14n = 84
n = 84/14 = 6