Relative Atomic Mass & Relative Molecular Mass (Oxford AQA International A Level Chemistry)

Relative Masses

Atomic Mass Unit

• The mass of a single atom is so small that it is impossible to weigh it directly

• Atomic masses are therefore defined in terms of a standard atom which is called the unified atomic mass unit

• This unified atomic mass is defined as one-twelfth of the mass of a carbon-12 isotope

• The symbol for the unified atomic mass is u (often Da, Dalton, is used as well)

• 1 u = 1.66 x 10^-27 kg

Relative atomic mass, A_r

• The relative atomic mass (A_r) of an element is the ratio of the average mass of the atoms of an element to the unified atomic mass unit

• The relative atomic mass is determined by using the average mass of the isotopes of a particular element

• The A_r has no units as it is a ratio and the units cancel each other out

Relative atomic mass of X =  

• Relative atomic mass can also be calculated from the abundance of different isotopes

Relative molecular mass, M_r

• The relative molecular mass (M_r) is the ratio of weighted average mass of a molecule of a molecular compound to the unified atomic mass unit

• M_r has no units

• M_r can be found by adding up the relative atomic masses of all atoms present in one molecule

• When calculating M_r the simplest formula for the compound is used, also known as the formula unit

  • Eg. silicon dioxide has a giant covalent structure, however the simplest formula (the formula unit) is SiO₂

Example M_r calculations

Relative formula mass, M_r 

• The term relative formula mass (M_r) is used for ionic compounds

• It has the same units and is calculated in the same way as the relative molecular mass

• In the table above, the M_r for potassium carbonate, calcium hydroxide and ammonium sulfates are relative formula masses