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 ]
Answer:
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 )
Answer:
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 )
Answer
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

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

Example:

The general formula of a hydrocarbon is C_nH_2n and the relative molecular mass of the hydrocarbon is 84. Find the value of n.

Answer:

Relative molecular mass of C_nH_2n
= n(12) + 2n(1) = 84
Hence

14n = 84
n = 84/14 = 6

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