Question

40 ml of hydrocarbon on combustion gave 120 ml $C{O}_2$ and 80 ml water vapour.
The molecular formula of hydrocarbon is?

Answer 1

$CxHy+O_2\to xC{O}_2+y2H_{2}O$
40 ml 120 ml 80 ml
Molar ratio of compound, $C{O}_2$ and water respectively is 1:3:2
3 moles of $C{O}_2$ contains 3 carbon atoms which should be equal to carbon number in the compound. Hence x=3
2 moles of $H_{2}O$ contains 4 hydrogen atoms which should be equal to hydrogen number in the compound. Hence the value of y is 4, The compound's molecular formula is $C_3{H}_4$.