Edexcel ·Combined Science·Cambridge IGCSE Combined Science

Stoichiometry & the Mole

14 min read

Relative atomic and formula mass, the mole, balancing equations, and calculations of mass and reacting quantities.

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

H_2O = (2\times1) + 16 = 18

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