Gaseous benzene reacts with hydrogen gas in presence of a nickel catalyst to form gaseous cyclohexane according to the reaction,
\[
C_6H_6(g) + 3H_2(g) \xrightarrow{Ni} C_6H_{12}(g)
\]
A mixture of \( C_6H_6 \) and excess \( H_2 \) has a pressure of 60 mm 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 \( C_6H_6 \) (by volume) present in the original volume is