MCQ Detailed View

A homologous series is a group of organic compounds having the same functional group, similar chemical properties, and a general formula where each successive member differs by a CH₂ unit in its molecular structure. The addition of this CH₂ group leads to a predictable and fixed increase in the molecular mass between adjacent members.

The molecular mass of a CH₂ group is calculated as follows:

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  Carbon (C) = 12 atomic mass units (amu)

  
  


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  Hydrogen (H₂) = 1 × 2 = 2 amu

  
  


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  Total mass of CH₂ = 12 + 2 = 14 amu

  
  

This means that each compound in a homologous series has a molecular mass 14 amu greater than the compound before it. For example:

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  Methane (CH₄) = 16 amu

  
  


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  Ethane (C₂H₆) = 30 amu

  
  


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  Propane (C₃H₈) = 44 amu

  
  

Each time, the difference in molecular mass is 14 amu due to the addition of one CH₂ unit. This pattern is consistent across many organic homologous series such as alkanes, alkenes, alcohols, and carboxylic acids, provided they belong to the same functional group family.

The chemical properties remain similar among members because the functional group, which determines reactivity, does not change. Physical properties like boiling point, melting point, and density show a gradual change due to the increase in molecular size and mass.

Therefore, the mass difference of 14 amu is a characteristic feature of homologous series and is an important concept in the classification and study of organic compounds.