Relative formula mass

Add the atomic masses

Relative formula mass, M_r, is the sum of the relative atomic masses, A_r, of every atom in a formula. These relative values have no unit. You can use “relative molecular mass” for molecules.

Use these relative atomic masses, A_r: H 1; C 12; N 14; O 16; Na 23; Mg 24; Al 27; S 32; Cl 35.5; K 39; Ca 40; Fe 56; Cu 64. Use values given in your exam question.

Water: count, multiply, add

H₂O has 2 H and 1 O.
M_r = (2 × 1) + 16 = 18.

Brackets must be multiplied too

Ca(OH)₂ contains 1 Ca, 2 O and 2 H.
M_r = 40 + (2 × 16) + (2 × 1) = 74.
The bracket means two whole OH groups, not just two H atoms.

Formula mass is not changed by a coefficient

M_r of water is always 18. 2H₂O contributes 2 × 18 = 36 to the relative mass total in an equation; it does not change water’s M_r.
For 2H₂ + O₂ → 2H₂O: (2 × 2) + 32 = 2 × 18 = 36. The mass totals match.