Fuels and combustion — chemical formula of iso-octane (the primary high-octane reference fuel) What is the correct molecular formula for iso-octane?

Introduction / Context:
Octane rating benchmarks knocking tendency using two reference fuels: n-heptane (low resistance to knock) and iso-octane (2,2,4-trimethylpentane, high resistance). Knowing iso-octane’s formula helps connect fuel chemistry to engine behavior.

Given Data / Assumptions:

• Iso-octane is an isomer of octane.
• Hydrocarbons follow C_n H_(2n+2) for alkanes (saturated).

Concept / Approach:
For an alkane with n = 8 carbon atoms, the general formula is C8H18. Iso-octane is one isomer with branching that improves knock resistance but retains the same molecular formula as straight-chain octane.

Step-by-Step Solution:

Verification / Alternative check:
Chemical data sheets list iso-octane (2,2,4-trimethylpentane) as C8H18 with molecular weight about 114.23 g/mol.

Why Other Options Are Wrong:

• C7H17 or C7H18: seven-carbon species, not octane.
• C6H18: violates alkane formula (for n=6, H=14).
• C8H16: corresponds to an alkene (unsaturated), not iso-octane.

Common Pitfalls:
Confusing structural isomerism (changes in bonding arrangement) with empirical/molecular formulas. All octane isomers share C8H18.

Final Answer: