General formulas, functional groups and chemical families
Use all four required general formulas, including the different carbon count in the carboxylic-acid notation.
Core and Extended · Chemistry 0620, 2026–2028. Structural isomers and the fuller homologous-series characteristics are taught in Extended lessons.
Before you begin
- Read subscripts and count atoms in a displayed formula.
- Identify C=C, C–O–H and the COOH group in supplied structures.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is a functional group's role?
What you will learn
- Write and interpret the required alkane, alkene, alcohol and carboxylic-acid general formulas.
- Explain a functional group and a homologous series through their similar chemical behaviour.
State the formulas with their counting convention
The required simple-series formulas are alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH and carboxylic acids CₙH₂ₙ₊₁COOH. In the first three, n counts all carbon atoms in the stated simple unbranched/acyclic examples. The simple alkene series has one C=C bond and starts at ethene, n=2; the stated alcohols have one OH group. The alcohol notation already has one extra hydrogen in OH: n=2 gives C₂H₅OH, total C₂H₆O.
In CₙH₂ₙ₊₁COOH, n counts the carbon atoms in the attached hydrocarbon part, and the COOH contributes another carbon. For ethanoic acid n=1 gives CH₃COOH, total C₂H₄O₂. For methanoic acid n=0 gives HCOOH. Do not set n=2 just because ethanoic acid has two carbons and then unintentionally describe a three-carbon acid. These are the specified simple-series patterns, not universal formulas for every possible carbon compound.
The family shares a chemical feature
A functional group is an atom or group of atoms that determines the chemical properties of a homologous series. The C=C feature is associated with alkenes, –OH with the required alcohols, and –COOH with carboxylic acids. Alkanes have the stated single-bond hydrocarbon pattern. A homologous series is a family with similar chemical properties due to the shared characteristic/functional group.
Similar chemical behaviour does not mean identical boiling points, identical atom totals or identical molecules. Recognise the group from connectivity as well as its symbols: an O–H inside a COOH group belongs to the carboxylic-acid group and is not enough to classify the molecule as the simple alcohol example. The Extended lesson develops the fuller sequence and physical-trend characteristics.
Pause and explain
What is the total molecular formula for C₂H₅OH?
Worked example
Use n=2 in the alkane and alcohol formulas, then use the correct n for ethanoic acid in the stated acid formula.
Show the worked solution
- Alkane CₙH₂ₙ₊₂ gives C₂H₆.
- Alcohol CₙH₂ₙ₊₁OH gives C₂H₅OH, total C₂H₆O.
- Ethanoic acid uses n=1 for its attached CH₃ part, giving CH₃COOH, total C₂H₄O₂.
Answer C₂H₆; C₂H₅OH (C₂H₆O); ethanoic acid CH₃COOH with n=1.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State all four required general formulas and identify the carbon-counting difference for the acid.
Give me a hint
Do not hide the COOH carbon when explaining n.
Compare my reasoning
- Alkanes CₙH₂ₙ₊₂ and alkenes CₙH₂ₙ use the total carbon count in the stated simple models.
- Alcohols CₙH₂ₙ₊₁OH add the OH hydrogen to the hydrocarbon-part count.
- Carboxylic acids CₙH₂ₙ₊₁COOH have n carbons in the attached part plus one in COOH.
Alkane CₙH₂ₙ₊₂; alkene CₙH₂ₙ; alcohol CₙH₂ₙ₊₁OH; acid CₙH₂ₙ₊₁COOH, with n+1 total carbons in the acid notation.
Look for these in your work
- Every required formula is correct.
- The acid and alcohol counting conventions are explained.
Find the simple alkane and alkene formulas for three carbons, the alcohol total for n=3, and the acid total for n=2.
Give me a hint
Use each formula independently and count the OH/COOH atoms too.
Compare my reasoning
- Alkane is C₃H₈; alkene is C₃H₆.
- Alcohol C₃H₇OH totals C₃H₈O.
- Acid C₂H₅COOH totals C₃H₆O₂ because COOH adds C, O₂ and H.
C₃H₈; C₃H₆; C₃H₈O; C₃H₆O₂ respectively.
Look for these in your work
- Each substitution uses the right series formula.
- Group atoms are included in totals.
A student uses n=2 in CₙH₂ₙ₊₁COOH and calls the result ethanoic acid because n is two. Correct the carbon count and give ethanoic acid's required notation.
Give me a hint
The named acid's total includes the COOH carbon.
Compare my reasoning
- n=2 gives C₂H₅COOH, containing three carbon atoms overall.
- Ethanoic acid has two carbons, so its attached part needs n=1.
- Its notation is CH₃COOH, molecular total C₂H₄O₂.
n=2 gives a three-carbon acid; ethanoic acid uses n=1 and CH₃COOH (C₂H₄O₂).
Look for these in your work
- The extra COOH carbon is counted.
- The corrected notation and molecular total agree.
Common mistakes
- Omitting the alcohol's OH hydrogen from its total.
- Counting COOH's carbon twice in the carboxylic-acid formula convention.
- Calling identical physical properties necessary for a chemical family.
- Classifying any O–H-containing structure as a simple alcohol without its context.
What can you explain now?
Which n gives ethanoic acid in CₙH₂ₙ₊₁COOH, and how many carbons are present overall?
Compare with the explanation
n=1; two carbon atoms overall.
The attached part has one C and COOH adds another.
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Make it stick
Write all four formulas without notes. Substitute fresh values and explain the alcohol hydrogen and carboxylic-acid carbon counts aloud.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.