Q .     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)(g) -> C_(6)H_(12)(g)`

A mixture of `C_(6)H_( 6)` and excess `H_(2)` has a
pressure of `60mm` of `Hg` in an unknown
volume. After the gas had 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 at the same
temperature. The fraction of `C_(6)H_( 6)` (by
volume) present in the original volume is