# 3.1.1 Relative Atomic Mass and Relative Molecular Mass

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## Relative Atomic Mass

### Relative Mass

The relative mass of an object is the comparison of the mass of the object to the mass of a standard object.

### Relative Atomic Mass

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.

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?

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)

Relative Atomic Mass of Element A = 2 x 12 = 24

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 )

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 )

2(80) / 4(20) =2

The mass of 2 bromine atoms is 2 times greater than the mass of 4 neon atoms.

Example 5
4 atoms of element L have the same mass as 1 tellurium atom. Find the relative atomic mass of L. (Ar: Te = 128 )

4L = 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 the mass of an atom of carbon-12, which taken as 12 units.

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 ]

The formula of carbon dioxide = CO2

Relative molecular mass of CO2 = 1 x 12 + 2 x 16 = 44

Example
What is the relative molecular mass of  aluminium sulphate [ Al2(SO4)3]?
( Ar: O = 16, S = 32; Al = 27 )

Relative molecular mass of Al2(SO4)3
= 2 x 27 + 3( 32 + 4 x 16) = 342

Example:
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 )

Let’s say the relative atomic mass of element X = m

The relative moleculaar mass of  KXO3

= 39 + m + 3(16) = 167
m = 167 – 39 – 3(16) = 80

The relative atomic mass of element X = 80
Example:
The general formula of a hydrocarbon is CnH2n and the relative molecular mass of the hydrocarbon is 84. Find the value of n.