Gaseous benzene reacts with hydrogen gas in presence of a nickel catalyst to form gaseous cyclohexane according to the reaction:
`C_(6)H_(6) (g) + 3H_(2) (h) rArr C_(6) H_(12) (g)`
A mixture of `C_(6)H_(6)` and excess `H_(2)` has a pressure of 60 mm of Hg in an unknown volume. After the gas has been passed over a nickel catalyst and all the benzene converted to cyclohexane, the pressure of the gas was 30 mm of Hg in the same volume and temperature. The fraction of `C_(6)H_(6)` (by volume) present in the original mixture is :

Answer

6