IGCSE Chemistry: Organic Chemistry — Formulae and Naming — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Organic Chemistry revision notes

Tier note: questions and parts marked (0620 Extended, 5070 required) go beyond 0620 Core: the full list of homologous-series characteristics, structural formulae and structural isomers (syllabus 11.1 outcomes 7–9), naming unbranched compounds beyond methane and ethane, including locants (11.2 outcome 3), empirical formula (3.1 outcome 5), addition as a reaction type (11.5 outcomes 5–6), intermolecular forces (2.5 outcome 5) and comparing the two ways of making ethanol (11.6 outcome 4). Everything else is 0620 Core. 5070 has no tiers, so O Level candidates need all of it.

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Questions

1. (0620 Extended, 5070 required) Define a homologous series, giving three characteristics. [3]

2. Give the general formula of the alkanes, alkenes and alcohols. [3]

3. (0620 Extended, 5070 required) Name and draw or describe the structure of the first four alkanes. [4]

4. (0620 Extended, 5070 required) Define a structural isomer and give the two structural isomers of C₄H₁₀ with their names. [3]

5. Describe how you would distinguish ethane from ethene.

(a) State the reagent and both observations. [3] (b) (0620 Extended, 5070 required) Explain why the alkene reacts. [2]

6. (Extension beyond this resource’s core scope — fractional distillation and cracking are covered in Petroleum, Alkanes and Alkenes ; included here only because crude oil supplies the alkane/alkene feedstocks named above.) Crude oil is separated by fractional distillation.

(a) Explain the principle on which the separation depends. [2] (b) (0620 Extended, 5070 required) Explain why fractions collected near the top of the column have lower boiling points. [2] (c) Explain why cracking is carried out. [3]

7. (Extension beyond this resource’s core scope — ethanol production is covered in Alcohols and Carboxylic Acids ; included here only as a natural follow-on from the alcohol general formula in question 2.) Ethanol can be made by fermentation.

(a) State the conditions required. [3] (b) Write the word equation. [1] (c) (0620 Extended, 5070 required) Give one advantage and one disadvantage compared with hydration of ethene. [2]

8. (0620 Extended, 5070 required) Propan-1-ol and propan-2-ol have the same molecular formula, C₃H₈O.

(a) Explain what a locant number tells you, using these two compounds as your example. [2] (b) State whether propan-1-ol and propan-2-ol are structural isomers of each other, giving a reason. [2]

9. For butane, C₄H₁₀:

(a) (0620 Extended, 5070 required) Give its empirical formula. [1] (b) Explain the difference between its molecular formula and its displayed formula. [2] (c) State one piece of information a displayed formula gives that a molecular formula does not. [1]

10. Ethene is bubbled through bromine water. State the colour change and name the type of reaction. (Naming the type of reaction is 0620 Extended, 5070 required.) [2]

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Answers

1. A family of compounds with the same general formula [1], the same functional group and similar chemical properties [1], and a gradual change in physical properties with each member differing by CH₂ [1].

2. Alkanes CₙH₂ₙ₊₂ [1]; alkenes CₙH₂ₙ [1]; alcohols CₙH₂ₙ₊₁OH [1].

3. Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀ [1] [1] [1] [1] — each a chain of carbons with single bonds and hydrogens filling the remaining bonds.

4. Compounds with the same molecular formula but different structural arrangement [1]. Butane — an unbranched four-carbon chain [1]; methylpropane — a three-carbon chain with a methyl branch [1].

5. (a) Add bromine water [1]. Ethene decolourises it from orange to colourless [1]; ethane produces no change [1]. (b) Ethene contains a C=C double bond [1] which can undergo an addition reaction with bromine [1].

6. (a) The fractions have different boiling points [1], so they condense at different heights in the column [1]. (b) Those fractions have shorter chains with fewer electrons, so weaker intermolecular forces [1], meaning they remain gaseous until they reach the cooler top of the column [1]. (c) There is greater demand for short-chain hydrocarbons such as petrol than the supply from distillation [1]; cracking converts long-chain, less useful fractions into shorter, more useful ones [1] and also produces alkenes needed for making polymers [1].

7. (a) Yeast [1], aqueous glucose at a temperature of 25–35 °C [1], and absence of oxygen (anaerobic conditions) [1]. (b) glucose → ethanol + carbon dioxide [1]. (c) Advantage: uses a renewable raw material [1]. Disadvantage: it is a slow batch process giving an impure, dilute product [1].

8. (a) A locant number shows where on the chain a feature (here, the –OH group) is positioned [1]: propan-1-ol has the –OH on an end carbon, while propan-2-ol has it on the middle carbon [1]. (b) Yes [1] — they share the same molecular formula (C₃H₈O) but have different structural arrangements, which is exactly the definition of a structural isomer [1].

9. (a) C₂H₅ [1] (the simplest whole-number ratio of C to H, 4:10 simplified to 2:5). (b) The molecular formula (C₄H₁₀) gives only the total number of each type of atom [1]; the displayed formula shows every individual atom and every bond drawn out, including all C–H bonds [1]. (c) It shows which isomer is being described — for example, whether the chain is straight (butane) or branched (methylpropane), which the molecular formula alone cannot distinguish [1]. (Also accept: it shows how the atoms are arranged and bonded to each other.)

10. Orange to colourless [1]; addition reaction (bromine adds across the C=C double bond) [1].

Tip: the mark scheme gives the change as “orange to colourless”; “clear” is not the same as colourless. Try the real question next: Cambridge O Level Chemistry 5070, June 2025, Paper 22, Question 3.

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Where marks are usually lost

• (0620 Extended, 5070 required) Giving only one characteristic of a homologous series.
• Saying the alkene “turns bromine water brown” — it decolourises it.
• Explaining fractional distillation without linking boiling point to chain length.
• Giving only one reason for cracking.
• (0620 Extended, 5070 required) Treating propan-1-ol and propan-2-ol as if they were the same compound, since they share a molecular formula — locant numbers exist precisely to distinguish them.
• (0620 Extended, 5070 required) Confusing empirical formula (simplest ratio) with molecular formula (actual number of atoms) — for a compound like C₄H₁₀ these are different, but for others (like water, H₂O) they happen to be identical.
• Omitting C–H bonds when drawing a displayed formula — every bond, not just the “interesting” ones, must be shown.

The four types of formula, side by side

Type	What it shows	Butane example
Empirical (0620 Extended, 5070 required)	Simplest whole-number ratio of atoms	C₂H₅
Molecular	Actual number of each atom in one molecule	C₄H₁₀
Structural (0620 Extended, 5070 required)	How atoms are grouped, without every bond drawn	CH₃CH₂CH₂CH₃
Displayed	Every atom and every bond drawn out in full	(all atoms and all bonds drawn out)

Examiners specifically ask for a displayed formula when they want to see the arrangement that distinguishes one isomer from another — a molecular formula on its own, such as C₄H₁₀, is genuinely ambiguous between butane and methylpropane, which is exactly the point being tested whenever a question pairs isomerism with a request to “draw” rather than just “name” a compound. For the full naming rules, including stem names and ester naming, see the Organic Chemistry revision notes.