Question

A hydrocarbon is burnt completely in air to form 13.2g of carbon dioxide gas and 7.2g of water. What is the molecular formula of the hydrocarbon?

Answer 1

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Answer 2

Since it is a hydrocarbon that burns and combines with oxygen in the air, so all the carbon in carbon dioxide derives from the hydrocarbon, and similarly for hydrogen in water.

Now for CO2,
proportion of carbon/CO2
=12/(12+2*16)
=12/44
=3/11
Mass of carbon in hydrocarbon
=mass of CO2 * 3/11
= 13.2 * 3/11
= 3.6 g

Similarly,
proportion of H2 in H2O
= 2*1/(2*1+16)
= 2/18
= 1/9

Mass of H2 in hydrocarbon
= 7.2 * 1/9
= 0.8 g

Ratio of number of carbon atoms to number of hydrogen atoms
= (3.6/12) / (0.8/1)
= 0.3 /0.8
= 3/8

So the empirical formula is
C3H8 or any multiple there of.