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💡 Add up the atoms
A molecule’s mass is just the sum of its atoms’ masses. Know the formula, add up the atomic masses, and you have the molecular mass.
What is molecular mass?
- The total mass of a molecule .
- Found by adding the atomic masses of all atoms in the formula.
- Only for substances that form molecules (covalent compounds).
Example 9.4 — Molecular mass of water (H₂O)
Atomic mass: H = 1 u, O = 16 u. Molecular mass of \( \text{H}_2\text{O} = (1\times 2) + (16\times 1) = 18\ \text{u} \).
Example 9.5 — Molecular mass of carbon dioxide
Atomic mass: C = 12 u, O = 16 u. Molecular mass of \( \text{CO}_2 = (12\times 1) + (16\times 2) = 44\ \text{u} \).
In ionic compounds the ions form 3-D crystals, so ionic compounds do not form molecules.
⏸▶ Pause and Ponder
- 21. Nitric acid HNO₃ \(= 1 + 14 + (16\times 3) = \mathbf{63}\ \text{u}\).
- 22. Methane CH₄ \(= 12 + (1\times 4) = \mathbf{16}\ \text{u}\).
Important Definitions
- Molecular mass — the total mass of a molecule, equal to the sum of the atomic masses of its atoms.
