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Relative formula mass (M_r) is mass of a molecule or compound (on a scale compared to carbon-12).

It is calculated by adding up the relative atomic masses (A_r) of all the atoms present in the formula.

Example:

The relative formula mass (M_r) for water (H₂O) is 18.

Water                     = H₂O

Atoms present      = (2 x H) + (1 x O)

M_r                           = (2 x 1) + (1 x 16) = 18