Assuming that petrol is octane $\left(C_8{H}_{18}\right)$ and has a density of $0.8gm/ml,1.425$ litres of petrol on complete combustion will consume

Petrol is assumed as octane having a chemical formula $C_8{H}_{18}$ and when this undergoes combustion it forms carbon dioxide and water.

The balanced chemical equation is

$2{\mathrm{C}}_8{\mathrm{H}}_{18}+25{\mathrm{O}}_2\to 16{\mathrm{CO}}_2+18{\mathrm{H}}_2\mathrm{O}$

 The density, $\mathrm{d}=0.8\mathrm{gm}/\mathrm{ml}$

The molar mass of ${\mathrm{C}}_8{\mathrm{H}}_{18}=114 \mathrm{g}/\mathrm{mol}$

Volume of petrol given $=1.425 L$

We know, density, $\mathrm{d}=$ mass/volume mass $=0.8\times 1.425\times 1000=1140 g$

And no. of moles $=$ given mass/molar mass

So, the no. of moles of octane $=1140/114=10$

2 moles of ${\mathrm{C}}_8{\mathrm{H}}_{18}$ requires 25 moles of $O_2$

10 moles of ${\mathrm{C}}_8{\mathrm{H}}_{18}$ requires $=\frac{25}{2}\times 10=125$

Hence, 10 moles of ${\mathrm{C}}_8{\mathrm{H}}_{18}$ requires 125 moles of $O_2$.