Lesson 6, Topic 5
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# Relative Molecular Mass

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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

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 x 3)

40 + 12 + 48 = 100

(b) HNO3

1 + 14 + (16 x 3)

15  + 48 =   63

(c) KCl

39 + 35.5 = 74.5

error: