Learning Objectives:• To understand what is meant by the term, ‘relative molecular mass’

What is Relative Molecular Mass

Relative Molecular Mass, Mr

Mr is the relative molecular mass of a compound.

This is the mass of the compound relative to the mass of 12C.

It is easy. Just add up the masses of the atoms in the molecule.

Masses of MoleculesExampleH2 contains 2 x hydrogen atoms.

Hydrogen atoms have a relative mass of 1, so H2 has a relative mass of:

H H HH

mass = 1 mass = 1 H2

2 x 1 = 2g

Masses of MoleculesExampleCO2 contains 1 x carbon atom and 2 x oxygen atoms.carbon atoms have a relative mass of 12, and oxygen atoms have a relative mass of 16.

The sum is: (1 x 12) + (2 x 16)= 12 + 32= 44g

CO O

16 12 16