Show that the role of cone, sulphuric acid is not a dehydrating agent, but used for production of nitronium cation. 

Explanation

Nitration mixture is a mixture of conc. nitric acid and conc. sulphuric acid.
Sulphuric acid acts as an acid and donates a proton to nitric acid.
`HHSO_4 + HNO_3 to HSO_4^(-) + H_2overset(+)(O)-NO_2`
The protonated nitric acid, gives nitronium cation by the loss of water molecule.
`H_2overset(+)(O)NO_2 to NO_2^(+) +H_2O`
Benzene gives nitorbenzene by electrophilic substitution.
`C_6H_6 + NO_(2)^(+) to C_6H_5NO_2 + H^(+)`
Sulphuric acid is obtained back from bisulphate ion.
`H^(+) +HSO_4^(-) to  H_2SO_4`