A quantity of electricity required to reduce 12.3 g of nitrobenzene...

Molar mass of nitrobenzene = 123 Equivalent mass of nitrobenzene
$\text{=}\dfrac{\text{Molecular}\text{mass}}{6}$
$= \dfrac{123}{6} = 20.5$
Number of equivalents of nitrobenzene
$\text{=}\dfrac{\text{Mass}}{\text{Equivalent}\text{mass}}$
$= \dfrac{12.3}{20.5} = 0.6$
Amount of charge required $= 0.6 \times 96500 = 57900$ Coulomb
Amount of charge required when current efficiency is 50%
$= 2 \times 57900$
$\text{=}\text{115800}\text{C}$