International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which of the following is the displayed formula for pentane?

1. C5H12

2. C4H10

3. C3H8

4. C2H6

The correct answer is: C5H12

The correct choice identifies pentane as C5H12, which is the molecular formula representing a straight-chain alkane containing five carbon atoms and twelve hydrogen atoms. In a displayed structural formula, each carbon atom is represented, along with the connections to hydrogen atoms that fill each carbon's valence shell. Pentane is categorized as an alkane because it contains only single bonds between the carbon atoms and is fully saturated with hydrogen. By having five carbon atoms, each carbon in the chain is bonded to enough hydrogen atoms to satisfy carbon's tetravalency, meaning it forms a total of four bonds. The other formulas listed correspond to different alkanes with fewer carbon atoms. For instance, C4H10 is butane, C3H8 is propane, and C2H6 is ethane. Each of these compounds has fewer carbon atoms than pentane, thereby having a different chain structure and a distinct formula altogether. Thus, the displayed formula for pentane is accurately represented by C5H12, making it the correct answer.