Petroleum fuels knowledge check: For typical straight-run gasoline (petrol) blended for spark-ignition engines, what is the approximate average molecular weight range of the overall mixture under standard refinery characterisation?

Introduction / Context:
Petrol (gasoline) is not a single compound but a complex mixture of hydrocarbons, mainly in the carbon number window C5 to about C12. Refiners often characterise such mixtures through average or pseudo-component properties, including average molecular weight, to guide design, blending, and engine performance expectations.

Given Data / Assumptions:

• Straight-run gasoline range roughly C5–C12 with paraffins, isoparaffins, naphthenes, and aromatics.
• Average properties are mixture-averaged and depend on crude source and processing severity.
• We seek a typical, practical range used in teaching and design problems.

Concept / Approach:
Molecular weight scales approximately with carbon number. For C5–C12 families, the molecular weights of the constituents span about 72 (pentanes) to 170+ (dodecanes/aromatics). A blend centred around C7–C9 will average near 100–120. Hence, an educationally accepted gasoline average molecular weight lies in the 100–130 band, allowing normal blending variability.

Step-by-Step Solution:

Verification / Alternative check:
Property tables for gasoline cuts in refinery handbooks routinely show average molecular weight values near 100–110 for common straight-run or mildly processed blendstocks, confirming the order of magnitude.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing component molecular weight with mixture average, or assuming a single fixed value rather than a practical range driven by crude and process severity.

Final Answer:
100–130