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Relative formula mass (M_r) is the sum of the relative atomic mass of all the atoms present in the formula of a substance. Covalent substances are molecules , so their relative formula mass is often referred to as relative molecular mass.

Example:

Calcium hydroxide, Ca(OH)₂ consists of one calcium atom (A_r = 40), two oxygen atoms (A_r = 16) and two hydrogen atoms (A_r = 1).

M_r of Ca(OH)₂ = 40 + 16 + 16 + 1 + 1 = 74

or

M_r of Ca(OH)₂ = 40 + 2(16 + 1) = 74