Consider the nitration of benzene using mixture of conc.${\mathrm{H}}_2{\mathrm{SO}}_4$ and ${\mathrm{HNO}}_3$. If a large amount of ${\mathrm{KHSO}}_4$ is added to the mixture, the rate of nitration will be

 Mechanism of nitration is : 

${\mathrm{HNO}}_3+2{\mathrm{H}}_2{\mathrm{SO}}_4⟶{\mathrm{NO}}_2^{+}+2{\mathrm{HSO}}_4^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

If a large amount of  ${\mathrm{KHSO}}_4$ is added then conc. of $\mathrm{HS}{{\mathrm{O}}_4}^{-}$ ions increases and the reaction will be shifted in backward direction hence, the rate of nitration will be slower.