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Chemistry · Ch 1 — Some Basic Concepts of Chemistry

Molecular Mass

1.7.3

Molecular Mass

Molecular Mass

Molecular mass is the sum of the atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together.

For example, a molecule of methane (CH4\text{CH}_4) contains one carbon atom and four hydrogen atoms. Its molecular mass is:

Molecular mass of CH4=(12.011 u)+4 (1.008 u)=16.043 u\text{Molecular mass of CH}_4 = (12.011\ \text{u}) + 4\,(1.008\ \text{u}) = 16.043\ \text{u}

Similarly, for water (H2O\text{H}_2\text{O}), which has two hydrogen atoms and one oxygen atom:

Molecular mass of H2O=2×atomic mass of hydrogen+1×atomic mass of oxygen\text{Molecular mass of H}_2\text{O} = 2 \times \text{atomic mass of hydrogen} + 1 \times \text{atomic mass of oxygen}

=2 (1.008 u)+16.00 u=18.02 u= 2\,(1.008\ \text{u}) + 16.00\ \text{u} = 18.02\ \text{u}

Note

The atomic masses used here (12.011 u for carbon, 1.008 u for hydrogen, 16.00 u for oxygen) are the average atomic masses of Section 1.7.2 — which is why they are not whole numbers.

A worked calculation on exactly this pattern — the molecular mass of glucose (C6H12O6C_6H_{12}O_6) — is the chapter's Problem 1.1; you will find it, solved step by step, on its own question page in this chapter's Q&A. …

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