Molecular Formula – IGCSE Chemistry Definition
IGCSE Chemistry definition of molecular formula: the actual number of atoms of each element in one molecule. Covers relation to empirical formula and calculation.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
The molecular formula tells you exactly how many atoms of each element are present in one molecule. It gives more information than the empirical formula, which only shows the simplest ratio. The 0620 syllabus (Supplement) requires you to determine molecular formulae from empirical formulae and relative molecular mass data.
The 0620 definition
The molecular formula shows the actual number of atoms of each element in one molecule of a substance.
Molecular formula vs empirical formula
| Feature | Empirical formula | Molecular formula |
|---|---|---|
| What it shows | Simplest whole-number ratio | Actual number of atoms per molecule |
| Example for ethane | CH₃ | C₂H₆ |
| Example for glucose | CH₂O | C₆H₁₂O₆ |
| Example for water | H₂O | H₂O (same) |
| Used for | Ionic compounds, initial calculations | Covalent/molecular compounds |
The molecular formula is always a whole-number multiple of the empirical formula.
How to determine the molecular formula
- Calculate (or be given) the empirical formula
- Calculate the empirical formula mass
- Divide the relative molecular mass (Mr) by the empirical formula mass to get the multiplier (n)
- Multiply every subscript in the empirical formula by n
Worked example
Empirical formula = CH₂O, Mr = 180
Empirical formula mass = 12 + 2(1) + 16 = 30
n = 180 / 30 = 6
Molecular formula = C₆H₁₂O₆ (glucose)
When are they the same?
The molecular formula equals the empirical formula when n = 1, meaning the simplest ratio already represents the actual molecule. Examples: H₂O, CO₂, NH₃, CH₄.
Worked exam question
A compound has the empirical formula CH₂. Its relative molecular mass is 56. Determine the molecular formula. (2)
Mark scheme
Empirical formula mass = 12 + 2(1) = 14 [1]; n = 56 / 14 = 4, so molecular formula = C₄H₈ [1]
Common exam mistakes
- Confusing molecular formula with empirical formula in answers. If a question asks for the molecular formula, giving CH₂O when the answer should be C₆H₁₂O₆ loses the mark.
- Using molecular formulae for ionic compounds. Ionic compounds do not form molecules — their formulae (e.g. NaCl, MgO) already represent the simplest ratio and are effectively empirical formulae.
- Forgetting to multiply all subscripts by n. Every element in the empirical formula must be multiplied, not just one.
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