This video steps through a very useful method used to calculate reacting masses.
1:26 calculate relative formula masses (including relative molecular masses) (Mᵣ) from relative atomic masses (Aᵣ)
Relative formula mass (Mr) is mass of a molecule or compound (on a scale compared to carbon-12).
It is calculated by adding up the relative atomic masses (Ar) of all the atoms present in the formula.
The relative formula mass (Mr) for water (H2O) is 18.
Water = H2O
Atoms present = (2 x H) + (1 x O)
Mr = (2 x 1) + (1 x 16) = 18
In Chemistry, the mole is a measure of amount of substance (n).
The abbreviation for mole is mol.
The mass of 1 mole of a substance is the relative formula mass (Mr) of the substance in grams.
The Mr of water is 18.
Therefore the mass of 1 mol of water equals 18 g.