Atoms, Molecules & Stoichiometry (the Mole)

Relative masses and the mole

Relative atomic mass $A_r$ — weighted mean mass of an atom relative to $\tfrac{1}{12}$ of a carbon-12 atom.
Relative molecular/formula mass $M_r$ — sum of the $A_r$ values.
The mole is the amount of substance containing the Avogadro constant $L = 6.02\times10^{23}$ particles.

$$n = \frac{m}{M} \qquad N = n \times L$$

$A_r$ from a mass spectrum: $A_r = \dfrac{\sum(\text{isotope mass} \times \text{abundance})}{\sum \text{abundance}}$.