Page "Stoichiometry" Paragraph 15

Different elements have a different atomic mass, and as collections of single atoms, molecules have a definite molar mass, measured with the unit mole ( 6. 02 × 10 < sup > 23 </ sup > individual molecules, Avogadro's constant ).

By definition, carbon-12 has a molar mass of 12 g / mol.

Thus to calculate the stoichiometry by mass, the number of molecules required for each reactant is expressed in moles and multiplied by the molar mass of each to give the mass of each reactant per mole of reaction.

The mass ratios can be calculated by dividing each by the total in the whole reaction.