What is the general formula for alkanes?

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Study for the IGCSE Edexcel Chemistry Test. Access flashcards and multiple choice questions with hints and explanations. Get ready for your exam!

Multiple Choice

What is the general formula for alkanes?

Explanation:
Alkanes are saturated hydrocarbons with only single bonds between carbon atoms, so each carbon forms four bonds. In a chain with n carbon atoms, the two end carbons contribute three hydrogens each, giving 6 hydrogens from the ends, while the remaining (n−2) interior carbons contribute two hydrogens each, giving 2(n−2) hydrogens. Total hydrogens = 2(n−2) + 6 = 2n + 2. So the general formula is CnH2n+2. This fits with simple examples like methane (CH4) when n = 1, ethane (C2H6) when n = 2, and propane (C3H8) when n = 3. Other formulas would imply unsaturation or rings and don’t match the valence of carbon in saturated, acyclic alkanes.

Alkanes are saturated hydrocarbons with only single bonds between carbon atoms, so each carbon forms four bonds. In a chain with n carbon atoms, the two end carbons contribute three hydrogens each, giving 6 hydrogens from the ends, while the remaining (n−2) interior carbons contribute two hydrogens each, giving 2(n−2) hydrogens. Total hydrogens = 2(n−2) + 6 = 2n + 2. So the general formula is CnH2n+2. This fits with simple examples like methane (CH4) when n = 1, ethane (C2H6) when n = 2, and propane (C3H8) when n = 3. Other formulas would imply unsaturation or rings and don’t match the valence of carbon in saturated, acyclic alkanes.

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