Bulk property of crude oils What is a representative range for the average molecular weight of whole crude oil as cited in introductory refining references?

Introduction / Context:Crude oil is a complex mixture of thousands of hydrocarbons. While individual molecules span a very wide range, an average molecular weight for the bulk crude is useful for rough calculations and for appreciating how crude heaviness varies across fields.

Given Data / Assumptions:

• We seek a commonly cited representative range (not an absolute for every crude).
• Introductory exam context favors mid-range values typical of medium crudes.
• The options bracket light to very heavy values.

Concept / Approach:Average molecular weight figures reported for many medium crudes often lie in the low-hundreds. Values near 230–250 are frequently quoted as representative. Heavier numbers (450–850) would suggest very heavy residues/asphalt fractions rather than whole-crude averages, while very low values (~100) correspond to narrow light fractions, not total crude.

Step-by-Step Solution:

Verification / Alternative check:Basic refining texts and problem books often adopt ~230 as a convenient average for worked examples.

Why Other Options Are Wrong:

• 100–120: Too low for whole crude; closer to light naphtha ranges.
• 450–550 / 600–850: More indicative of heavy fractions or residues, not whole-crude averages.

Common Pitfalls:Assuming one universal average for all crudes; the question asks for a representative range, not a constant.

Final Answer:230-250