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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?
Molecular Mass
The total mass of a molecule, found by adding the atomic masses of all its atoms. Water H2O equals 18 u
- 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 H 2 O = (1× 2) + (16× 1) = 18 u .
Example 9.5 — Molecular mass of carbon dioxide
Atomic mass: C = 12 u, O = 16 u. Molecular mass of CO 2 = (12× 1) + (16× 2) = 44 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× 3) = 63 u .
- 22. Methane CH₄ = 12 + (1× 4) = 16 u .
✅ Test Yourself
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How is molecular mass found?
Show Answer
By adding the atomic masses of all the atoms in the molecule. -
Find the molecular mass of water (H = 1 u, O = 16 u).
Show Answer
(1× 2) + (16× 1) = 18 u. -
Find the molecular mass of CO₂ (C = 12 u, O = 16 u).
Show Answer
(12× 1) + (16× 2) = 44 u. -
Do ionic compounds form molecules?
Show Answer
No — their ions form 3-D crystals instead.
Important Definitions
- Molecular mass — the total mass of a molecule, equal to the sum of the atomic masses of its atoms.
