Question

Hydrocarbon contains 18.18% hydrogen and 81.82% carbon. One litre of this gas has the same mass as carbon dioxide. What is the molecular formula of this hydrocarbon?

Answer 1

Given the percentage composition of HC as `C->81.82% and H->18.18%`

So the ratio of number if atoms of C and H in its molecule can will be

`C:H=81.82/12:18.18/1`

`C:H=6.82:18.18`
`=6.82/6.82:18.18/6.82=1:2.66~~3:8`

So the Empirical Formula of hydrocarbon is `C_3H_8`

As the mass of one litre of hydrocarbon is same as that of `CO_2` The molar mass of the HC will be same as that of `CO_2` i.e `44g/"mol"`

Now let Molecular firmula of the HC be `(C_3H_8)_n`

Using molar mass of C and H the molar mass of the HC from its molecular formula is
`(3xx12+8xx1)n=44n`

So

`44n=44=>n=1`

Hence the molecular formula of HC is `C_3H_8`