Assuming that petrol is octane $(C_{8} H_{18})$ and has a density of $0.8 g m / m l, 1.425$ litres of petrol on complete combustion will consume

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50 moles of $O_{2}$

100 moles of $O_{2}$

125 moles of $O_{2}$

200 moles of $O_{2}$

answer is C.

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Detailed Solution

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

$2 C_{8} H_{18} + 25 O_{2} \to 16 {CO}_{2} + 18 H_{2} O$

The density, $d = 0.8 gm / ml$

The molar mass of $C_{8} H_{18} = 114 g / mol$

Volume of petrol given $= 1.425 L$

We know, density, $d =$ 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 $C_{8} H_{18}$ requires 25 moles of $O_{2}$

10 moles of $C_{8} H_{18}$ requires $= \frac{25}{2} \times 10 = 125$

Hence, 10 moles of $C_{8} H_{18}$ requires 125 moles of $O_{2}$ .

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