IGCSE · Practice questions · Formulae, functional groups and homologous series

Formula, group, series.

Six original Cambridge-style questions on organic formulae, functional groups and homologous series.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Four formulae, three groups.

Molecular C₂H₆O, structural CH₃CH₂OH, displayed every bond.
Functional groups: C=C alkene, −OH alcohol, −COOH carboxylic acid.
Homologous series: same general formula and functional group, similar chemistry, trend in physical properties.
Saturated = C−C single bonds only; unsaturated = contains C=C.

Recall

[3 marks]

State three characteristics of a homologous series.

Members have the same general formula. ✓
They have the same functional group, so similar chemical properties. ✓
They show a trend (gradual change) in physical properties such as boiling point; consecutive members differ by CH₂. ✓

Recall

[2 marks]

Butane has the structural formula CH₃CH₂CH₂CH₃. Give its molecular formula and show that it fits the general formula of the alkanes.

C₄H₁₀. ✓
C_nH_2n+2 with n = 4 gives 2(4) + 2 = 10 hydrogen atoms. ✓

Apply

[2 marks]

Draw the displayed formula of ethanoic acid, CH₃COOH, and circle its functional group.

Three H atoms on the first carbon; the second carbon bonded to it, with a double bond to one O and a single bond to an O−H. ✓
Circle the whole −COOH group (C, the =O, and the O−H). ✓

Analysis

[3 marks]

Classify each compound by homologous series: (a) C₃H₆, a compound that decolourises bromine water; (b) C₂H₅OH; (c) C₃H₈.

(a) alkene (C_nH_2n, contains C=C): propene. ✓
(b) alcohol (−OH group): ethanol. ✓
(c) alkane (C_nH_2n+2): propane. ✓

Explain

[2 marks]

Explain why ethane is described as saturated and ethene as unsaturated.

Ethane contains only single carbon-carbon bonds. ✓
Ethene contains a carbon-carbon double bond, C=C. ✓

Data

[2 marks]

The boiling points of methane, ethane and propane are −162 °C, −89 °C and −42 °C. Describe the trend and predict whether butane is a gas at room temperature (20 °C).

Boiling point increases as the chain gets longer (each extra CH₂). ✓
The increases are getting smaller, so butane boils a little below 0 °C (actual value about −1 °C): it is a gas at 20 °C. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.