Molecular Mass

Just as relative atomic mass compares an atom's mass to the unified atomic mass unit, the relative molecular mass of a compound compares the mass of one molecule of it to the same unified atomic mass unit. Because a molecule is simply a fixed collection of atoms bonded together, its relative molecular mass is calculated by adding up the relative atomic masses of all the atoms in its formula.

Worked example 1 -- hydrogen molecule (H₂):

Relative molecular mass = 2 × (relative atomic mass of H) = 2 × 1.008 = 2.016 u

Worked example 2 -- glucose (C₆H₁₂O₆):

Relative molecular mass = (6 × 12) + (12 × 1.008) + (6 × 16) = 72 + 12.096 + 96 = 180.096 u …