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What is the general molecular formula for the homologous series of alkanes?
ACnH2n+2
BCnH2n
CCnHn
DCnH2n-2
Answer & Solution
Correct answer: A. CnH2n+2
1. Alkenes follow CnH2n and alkynes follow CnH2n-2.
2. Alkanes are saturated, so they carry two more hydrogens than the corresponding alkene.
3. This gives the general formula CnH2n+2.
4. Check: for n = 2 this gives C2H6 (ethane), which is correct.
5. Rule out B: CnH2n is the alkene formula, with one degree of unsaturation.
6. Rule out D: CnH2n-2 is the alkyne formula, with two fewer hydrogens.
7. Rule out C: CnHn has far too few hydrogens to satisfy carbon's tetravalency.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 9 "Carbon Compounds", p.129_
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