Question

complete combustion of a 0.0100mol sample of hydrocarbon, CxHy, gives 1.344 L of CO2 at STP and 0.720 g of H2O.
a. what is the molecular formula of hydrocarbon?
b. what is the empirical formula of hydrocarbon?..
1.344 L CO2 for 0.0100 mol sample would have evolved 134.4 L CO2 for a 1.00 mole sample. 134.4/44 = 3.05 moles C/mole unknown. So the formula is C3 H something.
0.720 g H2O for 0.01 mole would have evolved 72.0 g for a 1.00 mole sample and 72.0/18 = 3.99 moles H2O or that x 2 = 7.99 moles H atoms. So the molecular formula is C3H8 (3.05 rounds to 3.00 and 7.99 rounds to 8.00).

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