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10cm3 of a pure hydrocarbon was completly combusted in 70cm3 oxygen (excess).
The final gaseous mixture was 30cm3 Carbon dioxide and 20cm3 unreacted oxygen.
What is the formula of the hydrocarbon?
You know that the oxygen is in excess so the limiting agent is the hydrocarbon.
Take 10 cm³ to be proportional to 1 mol.
You know that for a complete combustion : hydrocarbon + AOxygen - > Bcarbon dioxide + Cwarter where A, B and C are the relative moles of each substance in the reaction.
For an incomplete combustion: hydrocarbon + DOxygen - > Ecarbon monoxide + Fwater where D, E and F are the relative moles of each substance in the reaction.
Balance the constants to get the hydrocarbon.