Answer:
Volume of 1 mole of a gas at STP =22.4L
Volume of 15 mole of a gas at STP =22.4×15=336L
Molecular weight of benzene =78g
No. of moles of benzene in 39g of benzene =
78
39
=0.5 mole
From the given reaction,
Volume of oxygen required to burn 2 mole of benzene =336L.
Therefore,
Volume of oxygen required to burn 0.5 mole of benzene =
2
336
×0.5=84L
Hence 84 litres of O
2
at STP are needed to complete the combustion of 39g of liquid benzene.