To convert between formula representations, count atoms carefully and keep the bonding rules in mind. A structural formula becomes a molecular formula by totalling each element, and a molecular formula becomes a structure only when the question also tells you the family or group.
This lesson finishes organic families and naming. It uses ideas from drawing a small molecule with correct bonding and formulas and equations.
What are the main representations?
For the same molecule, propan-1-ol, four forms are in common use:
| Form | What it shows | Propan-1-ol |
|---|---|---|
| Molecular formula | number of each atom | C₃H₈O |
| Structural formula | atoms grouped along the chain | CH₃CH₂CH₂OH |
| Displayed formula | every atom and bond drawn | see the drawing below |
| Name | family and chain length | propan-1-ol |
H H H
| | |
H - C - C - C - O - H
| | |
H H HThe general formulae fit alongside: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ. Exact forms and the level of detail you need depend on your syllabus year, so check the Cambridge page.
How do you convert, step by step?
Structural to molecular:
- Count the carbons in every group (each C in CH₃, CH₂, CH, COOH).
- Count the hydrogens in every group, including the H in -OH and -COOH.
- Count the oxygens.
- Write the formula as C, then H, then O.
Molecular to structural: use the family or name given, lay out the chain, add the group, then check that each carbon has four bonds.
Worked example
Convert CH₃CH₂CH₂OH to a molecular formula, then write the structural formula of butanoic acid and find its molecular formula.
Part 1. Carbons: CH₃, CH₂ and CH₂ give 3. Hydrogens: 3 + 2 + 2 + 1 (the H of -OH) = 8. Oxygens: 1. The molecular formula is C₃H₈O.
Part 2. Butanoic acid has four carbons and ends in -COOH, so the carboxyl carbon is the fourth carbon. The other three carbons form CH₃CH₂CH₂, giving the structural formula CH₃CH₂CH₂COOH.
Count for the molecular formula.
Carbons: 3 + 1 = 4. Hydrogens: 3 + 2 + 2 + 1 = 8.
Oxygens: 2. So butanoic acid is C₄H₈O₂.
Check against the bonding rules. The chain has 3 carbons in CH₃CH₂CH₂ plus the carboxyl carbon, and each carbon has four bonds, so nothing is missing.
The mistake to watch for
The most common error is forgetting the hydrogen inside -OH or -COOH, or miscounting a bracketed group.
Mistaken answer: CH₃CH(CH₃)CH₃ is C₄H₉.
The student counted 3 + 3 + 3 = 9 hydrogens and skipped the single H on the middle CH.
The correction is to write the count group by group: CH₃ gives 3, CH gives 1, the bracketed CH₃ gives 3 and the last CH₃ gives 3, a total of 10, so the formula is C₄H₁₀. A quick sense check also helps. A saturated four-carbon hydrocarbon must be C₄H₁₀, because 2n + 2 = 10.
Check yourself
1. Write the molecular formula of CH₃COOH.
Show answer
Carbons: CH₃ gives 1 and COOH gives 1, so 2. Hydrogens: 3 + 1 = 4. Oxygens: 2. The formula is C₂H₄O₂.
2. Write the structural formula of but-1-ene, a four-carbon alkene with the double bond between the first two carbons, and its molecular formula.
Show answer
Structural formula: CH₂=CHCH₂CH₃. Hydrogens: 2 + 1 + 2 + 3 = 8. The molecular formula is C₄H₈, which fits CₙH₂ₙ for n = 4.
3. A molecule has the molecular formula C₃H₈O. Can you write one single structure from that formula alone? Explain.
Show answer
No. C₃H₈O matches more than one structure, including propan-1-ol and propan-2-ol, which are isomers. You need extra information, such as the name or where the -OH sits, to choose one.
Where this leads next
You now have the working tools for the whole module. Test them together in the organic families and naming practice set, then move on to organic reactions. The mole and equation-ratio tutor is useful when formulae feed into calculations.
Students who can convert one formula but lose accuracy across a long question often gain from online one-to-one Chemistry tuition, where a teacher can check your counting step by step.