2C6H5+15O2→12CO2+6H2O
156 gm →480 gm
So for combustion of 39 g of C6H5,O2 required
=480×39156
=120 gm O2
So volume of =120 gm O2 at STP =22.4×12032
=84 ltr of oxygen
Therefore =84 ltr of oxygen is what is required to completely burn 39 g of liquid benzene.