F4chemistry

Relative molecular mass is usually used to represent the relative mass of a substance with molecular structure. The relative molecular mass of such a substance is the relative mass of a molecule of this substance.

Formula mass a usually used to represent the relative mass of a substance which does not have molecular structure or empirical formula is always used to represent the substance. The formula mass of such a substance is the relative mass of a formula unit of this substance.

For example:
(i) Carbon dioxide has molecular structure, with molecular formula CO2.
(i) Therefore, relative molecular mass of CO2 = 12 + 16x2 = 44

(ii) Sodium chloride has giant ionic structure, which does not contain any molecule.
(ii) Its empirical formula is NaCl, and each formula unit contains 1 Na+ and 1 Cl- ions.
(ii) Therefore, formula mass of NaCl = 23 + 35.5 = 58.5

(iii) Silicon dioxide (quartz) has giant covalent structure, which does not contain any molecule.
(iii) Its empirical formula is SiO2, and each formula unit contains 1 Si and 2 O atoms.
(iii) Therefore, formula mass of SiO2 = 28.1 + 16x2 = 60.1

(iv) Sulphur exists as S8 molecules, but its empirical formula S is always used to represent it.
(iv) Relative molecular mass of S8 = 32.1 x 8 = 256.8
(iv) Formula mass of S = 32.1