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

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — 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

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Questions

1. Define a homologous series, giving three characteristics. [3]

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

3. Name and draw or describe the structure of the first four alkanes. [4]

4. 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) 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) 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) Give one advantage and one disadvantage compared with hydration of ethene. [2]

8. 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) 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]

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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].

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Where marks are usually lost

• 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.
• 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.
• 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	Simplest whole-number ratio of atoms	C₂H₅
Molecular	Actual number of each atom in one molecule	C₄H₁₀
Structural	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.