Stoichiometry

Relative Atomic Mass (Ar):

• Relative atomic mass (Ar) is the average mass of an atom of an element compared to the mass of a carbon-12 atom.
• The mass of a carbon-12 atom is defined as exactly 12 units.
• Relative atomic masses are used to calculate the masses of elements in chemical reactions and equations.

Relative Molecular Mass (Mr):

• Relative molecular mass (Mr) is the sum of the relative atomic masses of the atoms in a molecule.
• Mr is used to calculate the masses of molecules in chemical reactions and equations.
• For ionic compounds, the relative formula mass (Mr) is used instead of relative molecular mass.

Example:

Calculate the relative molecular mass of water (H2O).

• Relative atomic mass of hydrogen (H) = 1.008
• Relative atomic mass of oxygen (O) = 16.00
• Relative molecular mass of water (H2O) = 2 × 1.008 + 16.00 = 18.02**