Relative Molecular Mass

The relative molecular mass of an element or a compound is the number of times the average mass of one molecule of it is heavier than one-twelfth the mass of one atom of carbon-12.

\(\scriptsize Relative \:Molecular \:Mass = \normalsize \frac{mass \: of \: 1 \: molecule \: of \: substance}{\frac{1}{12} \: \times \: mass \: of \: Carbon-12 } \)

The relative molecular mass of an element or compound is the sum of the relative atomic mass of the atoms in one molecule of that substance.

Example 6.5.1:

Calculate the relative molecular mass (RMM) of the following compounds.

(a) CaCO₃ (b) HNO₃ (c) KCl.

(Ca = 40, C = 12, O = 16, H = 1, N =14, K = 39, Cl = 35.5)

Solution:

(a) CaCO₃

40 + 12 + (16 × 3)

40 + 12 + 48 = 100

(b) HNO₃

1 + 14 + (16 × 3)

15  + 48 =   63

(c) KCl

39 + 35.5 = 74.5