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.


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