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Iso octane is

1. C8H18
2. C10H22
3. C12H26
4. C15H32

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Correct Answer - Option 1 : C8H18


The chemical formula of iso-octane is C8H18

Octane Rating:

The S.I. engine fuel has been given an anti-knocking rating which is known as Octane Rating. It basically shows the resistance of the fuel to create knocking by auto-ignition.

According to standard practice, the antiknock value of an SI engine fuel is determined by comparing its antiknock property with a mixture of two reference fuels, normal heptane (C7H18) and iso-octane (C8H18).

Iso-octane (C8H18) chemically being a very good antiknock fuel, is arbitrarily assigned a rating of "100 octane number".

Normal heptane (C7H16) has very poor antiknock qualities and is given a rating of "0 octane number".

Thus, if the content of Iso-octane is high in the fuel then it will ensure high octane number resulting in better fuel anti-knock quality. Because of the high Octane number, the fuel will not auto-ignite as the possibility of knocking has been diminished by the fuel quality.

Hence, Iso-octane content in fuel for S.I engine retards auto-ignition.

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