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Octane Formula – Structure, Properties, Uses, Sample Questions

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  • Last Updated : 31 Mar, 2022
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Octane is a hydrocarbon made up of carbon and hydrogen atoms. The formula for octane is C8H18. Most of the hydrocarbons groups are found in petroleum. Octane has many structural isomers but they are different depending on the amount and location of branching in the carbon chain. It is found in crude oil and is discovered by Edwin L. Drake in August 1859.


Carbon is a chemical element represented by the symbol is C. Its atomic number is 6. The electron structure of carbon is 1s2 2s2 2p2 with four valency electrons. It is an important element because it plays a very important role in real life. Carbon can be found in all living structures. Its compounds are found everywhere in real life ex: pains, medicines, plastics, cotton, wool, coal, etc. 


Hydrogen is a chemical element represented with the symbol H and its atomic number is 1. This element is discovered by Henry Cavendish in the year of 1766. Naturally, hydrogen exists in molecular form, it is the most abundant element on the earth. It is the lightest element and can be used in fuel cells to generate power. Hydrogen is essential for life without this element we wouldn’t have the sun to give us light and heat. It has three isotopes they are hydrogen, deuterium, tritium.


Octane is a hydrocarbon and alkane with the chemical formula C8H18. It is a chemical substance that exists in petrol or gasoline and that is used to measure the quality of fuel. It is a colorless liquid with the odor of gasoline, it is a volatile and highly flammable colorless liquid. its ratings are measured by a fuel’s ability to avoid knock, the knock occurs when fuel is permanently ignited in the engine’s cylinder, which degrades efficiency and can be damaging to the engine, the fuels contain an oxygenate that prevents knock by adding oxygen to the fuel, this oxygenate is commonly referred to as octane.

Structure of Octane

The condensed structural formula for octane is CH3(CH2)6CH3.


 Physical Properties

  • It has a molar weight of 114.23 g/mol
  • Its boiling point is 125.6°C
  • The melting point of Octane is -57°C
  • Its density is 703 kg/m3
  • It is a highly flammable colorless liquid.
  • It is a colorless liquid with an odor of gasoline

Chemical Properties

Hydrocarbons are found in nature in many forms mainly as liquid petroleum, it is known as crude oil to distinguish it from refined oil it is the most important commercially natural gas which is a lighter fraction of hydrocarbons can be free or dissolved asphalt, tar, the pitch is solid or semi-solid forms of hydrocarbons the heavy fraction.

Uses of Octane

  1. It is used as a solvent.
  2. It is used as a chemical raw material.
  3. It is used as an important chemical agent in the petroleum industry.
  4. It is used as a component of gasoline.
  5. The octane number is used to determine the quality of fuel.

Sample Questions

Question 1: How many isomers are in Octane?


Octane has 18 isomers. They are Octane, 2-Hethylheptane, 3-Hethylheptane, 4-Hethylheptane, 2,2-Dimethylhexane, 2,3-Dimethylhexane, 2,4-Dimethylhexane, 2,5-Dimethylhexane, 3,3-Dimethylhexane, 3,4-Dimethylhexane, 3-Ethylhexane, 2,2,3-Trimethylpentane, 2,2,4-Trimethylpentane, 2,3,3-Trimethylpentane, 2,2,4-Trimethylpentane, 2-Methyl-3-etylpentane, 3-Methyl-3-etylpentane, Tetramethylbutane, 15 of them are positional isomers and 3 of them stereoisomers.

These isomers are differed by the amount and location of branching in the carbon chain.

Question 2: Which compound is used to calculate the cetane number? 


The cetane molecule is used to calculate the cetane number its formula is C16H34. It is a saturated hydrocarbon and has a very short ignition lag as compared to any commercial deiseal fuel. Its cetane number is 100. Cetane Number is the percentile of cetane in a mixture of cetane and α-methyl naphthalene.

Question 3: Which is used to reduce the surface tension in deiseal fuel?


The chemical addictive’s are used to reduce the surface tension thus promoting the formation of finer spray .inhibitors are used to delay or prevent gum formation and poly hydrocarbons are used to preserve the fluid properties.

Question 4: which compound is used to calculate the octane number? 


An octane number is a number that is used to determine the quality of petrol fuel. Two pure hydrocarbons have been selected slandered by edger

  1. n-heptane
  2. Iso-octane

The octane number of iso-octane is 100 and the octane number of n-heptane is 0. The octane number of a fuel is numerically taken as the percentage of iso-octane in a mixture of iso-octane and n-heptane.

Question 5: What is the molar mass of octane? How to calculate?


The molar mass of octane is 114.23 g/mol. The carbon atomic mass is 12.0107. The number of atoms in carbon is 8. The atomic mass of hydrogen is 1.00794. The number of atoms in hydrogen is 18.

Finally, molecular mass calculation is = 12.0107 × 8 + 1.00794 × 18

= 96.0856 + 18.14292

= 114.22852

Question 6: What type of bond is Octane?


Octane belongs to a family of molecules called alkanes. These are hydrocarbons molecules with just single, two-electron bonds connecting the atoms. There is a whole series of them with increasing numbers of carbon atoms.

Examples of alkanes are methane, ethane, propane, etc.

Question 7: What is Knocking?


It is a sharp rattling sound produced in the internal combustion engine due to immature ignition of the air gasoline mixture the pressure of some constituents in the gasoline used causes the rate of oxidation to become so great that a lost portion of the fuel-air mixture gets ignited instantaneously producing explosive violence its called Knocking.

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