(i) Draw the structures of all possible structural isomers of pentane (C₅H₁₂). How many structural isomers are possible?
(ii) State the two characteristic properties of carbon responsible for forming such a large number of compounds.
(iii) Among the homologous series of alkanes, how do the following properties change as the molecular mass increases? Give a reason.
(a) Boiling point
(b) Chemical properties
(iv) Write the general formula for alkanes and use it to find the molecular formula of the alkane with 7 carbon atoms.
Generated by claude-sonnet-4-6 · 2026-06-26 01:08 · grounding rag
Model Answer
(i) Structural Isomers of Pentane (C₅H₁₂):
Three structural isomers are possible:
- n-Pentane (straight chain):
C—C—C—C—C
- Isopentane (one branch):
```
C
|
C—C—C—C
```
- Neopentane (two branches):
```
C
|
C—C—C
|
C
```
Total: 3 structural isomers
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(ii) Two characteristic properties of carbon:
- Catenation – Carbon atoms can bond with other carbon atoms to form long chains, branched chains, and rings.
- Tetravalency – Carbon has a valency of four, allowing it to bond with four other atoms simultaneously, forming a large variety of compounds.
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(iii)
(a) Boiling point increases with molecular mass, because larger molecules have stronger intermolecular forces, requiring more energy to overcome.
(b) Chemical properties remain similar within a homologous series, because all members have the same functional group (or none, for alkanes).
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(iv) General formula of alkanes: CₙH₂ₙ₊₂
For 7 carbon atoms (n = 7):
C₇H₂(₇)+₂ = C₇H₁₆ (Heptane)
Source: Carbon and its Compounds, Sections 4.2.2 and 4.2.5
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Explanation
- For part (i), CBSE expects you to draw all three skeletal/structural forms clearly — examiners award 1 mark per correct structure and 1 mark for stating the count.
- For part (ii), the exact terms catenation and tetravalency must appear — these are directly from the "What you have learnt" summary.
- For part (iii), state the trend AND give a reason — both are needed for full marks.
- For part (iv), memorise the general formula CₙH₂ₙ₊₂ and show substitution of n=7 explicitly.