100 ml of a hydrocarbon on combustion gave 300 ml of $C O_{2}$ and 400 ml of water vapor measured under similar conditions of temperature and pressure. Find out the molecular formula of hydrocarbon.

C_{2} H_{4}

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C_{3} H_{6}

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C_{3} H_{8}

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C_{3} H_{4}

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Solution

The correct option is D $C_{3} H_{8}$
They key here is that 1 mole of an ideal gas occupies the same volume as 1 mole of any other ideal gas under the same conditions of temperature and pressure. So, since the same volume of an ideal gas has the same number of moles then you can treat the ratio of volumes as to the ratio of moles.
all the Carbon from the hydrocarbon is in the $C O_{2}$
100 ml of a hydrocarbon produces 300 ml $C O_{2}$
$1 : 3$ ratio
So 1-mole hydrocarbon has 4 moles Carbon

All the H ends up in the water
100 ml hydrocarbon produces 400 ml of water,
$1 : 4$ ratio,
but also each water has 2 $H$ atoms
So 1-mole hydrocarbon has 8 moles Hydrogen

which gives
$C_{3} H_{8}$
answer is C

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100 ml of a hydrocarbon on combustion gave 300 ml of CO2 and 400 ml of water vapor measured under similar conditions of temperature and pressure. Find out the molecular formula of hydrocarbon.

100 ml of a hydrocarbon on combustion gave 300 ml of $C O_{2}$ and 400 ml of water vapor measured under similar conditions of temperature and pressure. Find out the molecular formula of hydrocarbon.