Unsaturated hydrocarbons
3 marks · NormalAn unknown hydrocarbon has the molecular formula C3H6.
Explain why this compound must be an alkene rather than an alkane.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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Must show: States that alkanes have formula CnH2n+2 or gives C3H8 for propane, States that alkenes have formula CnH2n or gives C3H6 for propene, Explains the difference based on hydrogen count or formula
Common mistakes: Mistaking the formula for an alkane; Not comparing with alkane and alkene formulae
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Marking points
M1 · 1 mark · States the general formula of alkanes (CnH2n+2) and/or gives C3H8 for propane.
M2 · 1 mark · States the general formula for alkenes (CnH2n) and/or gives C3H6 for propene.
A1 · 1 mark · Concludes that C3H6 matches the alkene formula, not the alkane formula, so it must be an alkene.
Worked solution
Alkanes have the general formula CnH2n+2. For n = 3, the alkane would be C3H8 (propane).
Alkenes have the general formula CnH2n. For n = 3, the formula is C3H6 (propene).
Because the compound is C3H6, it fits the alkene formula and not the alkane formula, so it must be an alkene.