Stoichiometry & the Mole

Relative masses

Relative atomic mass ($A_r$) — the average mass of an atom compared with $\tfrac{1}{12}$ of a carbon-12 atom.
Relative formula/molecular mass ($M_r$) — the sum of the $A_r$ values in a formula. For example, $M_r$ of $H_2O = (2\times1) + 16 = 18$.