The Consecutive members of an alkane homologous series differ by

Analyze the question: It asks what difference exists between consecutive members of the alkane homologous series.

Concept: A homologous series is a family of organic compounds with the same functional group and similar chemical properties. They share a general formula, and each member differs from the next by a constant unit, -CH₂- (a methylene group), which has a relative mass of 14.

Solution:

The general formula of alkanes is CₙH₂ₙ₊₂.

| n | Alkane | Formula | |---|--------|---------| | 1 | Methane | CH₄ | | 2 | Ethane | C₂H₆ | | 3 | Propane | C₃H₈ | | 4 | Butane | C₄H₁₀ |

Going from CH₄ to C₂H₆: the difference is C₂H₆ − CH₄ = CH₂. Going from C₂H₆ to C₃H₈: the difference is C₃H₈ − C₂H₆ = CH₂.

Each step adds one carbon and two hydrogens.

Checking the other options:

• A. CH: this does not match the pattern, since two hydrogens are added each time, not one.
• C. CH₃: this would add three hydrogens, which is wrong.
• D. CₙHₙ: this is not a fixed unit, so it cannot be a constant difference.

Answer: B. CH₂

Exam tip: Remember "Homologous = CH₂ apart", with a mass difference of 14. This applies to alkanes, alkenes, alkynes, and alkanols alike, so it is a quick mark in JAMB.