Octane Scale – Standard Value for Iso-octane (C8H18) On the primary reference fuel octane scale, what octane number is assigned to iso-octane (2,2,4-trimethylpentane, C8H18)?

Introduction / Context:
The octane number quantifies a spark-ignition fuel’s resistance to knock. It is defined by comparing a test fuel to blends of two primary reference fuels in a standard CFR engine. Knowing the assigned numbers for these references is basic and critical for interpreting fuel specifications at the pump and for engine calibration.

Given Data / Assumptions:

• Primary reference fuels: iso-octane and normal heptane.
• Standard test conditions and procedures (RON or MON) apply.
• Octane number is an index, not a direct physical property.

Concept / Approach:

By definition, iso-octane is assigned an octane number of 100, while normal heptane is assigned 0. A test fuel that behaves like a blend of X percent iso-octane and (100 − X) percent n-heptane will have an octane number X. Thus, iso-octane anchors the high end of the traditional scale.

Step-by-Step Solution:

Verification / Alternative check:

Fuel standards and engine testing literature consistently cite iso-octane as the 100 point, confirming this textbook value.

Why Other Options Are Wrong:

Values 0 or 50 contradict the defined scale; 120 exceeds the classic primary reference fuel range; 95 is a common pump octane for commercial fuels, not the reference assignment.

Common Pitfalls:

Confusing research octane number (RON) with pump octane (R+M)/2; the reference assignments remain the same for both testing methods.

Final Answer:

100