3.1.2 Amount of Substance - Relative atomic mass and relative molecular mass


Students should:
  • be able to define relative atomic mass (Ar) and relative molecular mass (Mr) in terms of 12C. (The term relative formula mass will be used for ionic compounds)

Relative atomic mass scale

The mass of atoms and other small particles is measured on the atomic mass scale as a relative value compared to a carbon atom. A carbon 12 atom is assigned a value of exactly 12.0000 on this scale and everything else is measured with respect to it.

Carbon atom Magnesium atom Hydrogen atom
mass = 12 mass = 2 x carbon atom mass = 1/12 x carbon atom

The magnesium atom has a mass twice that of the carbon atom, therefore a magnesium atom has a relative mass of 24.

3 helium atoms have a mass equal to the mass of a carbon atom, i.e. each helium atom has a relative atomic mass of 4. The relative atomic mass may be abbreviated to Ar or RAM.


Relative molecular mass

Molecules are just groups of atoms and as such they are also measured on the relative mass scale with carbon once again being the reference atom

Hydrogen atom Oxygen atom water molecule
mass = 1 mass = 16 mass = (2 x 1) + 16 = 18

A water molecule has a mass of 3/2 times that of a carbon atom, therefore it has a relative molecular mass of 18.

The relative molecular mass is calculated by adding up the relative masses of all of the atoms in a molecule of that substance.

All masses are measured relative to the mass of a 12C isotope = 12.0000 atomic mass units.

As relative molecular mass is a comparative measure it has no units.

Example: Benzene has the molecular formula C6H6

A benzene molecule contains 6 carbon atoms and 6 hydrogen atoms.

The relative molecular mass = (6 x relative atomic mass of carbon) + (6 x relative atomic mass of hydrogen)

The relative molecular mass of benzene = (6 x 12) + (6 x 1) = 78

The relative molecular mass is abbreviated as RMM or Mr.


Relative formula mass

For substances that are not simple molecular in nature, such as giant molecular structures or ionic compounds, the formula is taken as the simplest possible ratio of particles in the compound. The relative formula mass is the sum of the relative masses of the particles in the simplest ratio.

Example: Copper(II) nitrate has the formula Cu(NO3)2

Relative masses: Cu 63.5; N 14; O 16.

Relative formula mass = 1 x 63.5 + 2[14 + (3 x 16)] = 187.5


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