LPG volatility benchmark: At atmospheric pressure (760 mm Hg), the temperature at which about 95% by volume of LPG will evaporate is closest to which value?

Introduction:
Liquefied petroleum gas (LPG) is a mixture of light hydrocarbons (commonly propane and butane). Its volatility characteristics are important for storage, vaporisation, and appliance performance. A common benchmark is the temperature at which a specified percentage evaporates at 1 atm.

Given Data / Assumptions:

• Pressure: 760 mm Hg (atmospheric).
• Target: Approximately 95% by volume evaporated.
• Typical LPG composition: propane-rich blends for colder climates, varying with region.

Concept / Approach:
Propane has a normal boiling point near −42°C, while n-butane boils near −0.5°C. A propane-leaning LPG blend shows high volatility at sub-zero temperatures. The 95% evaporated point at atmospheric pressure is therefore near the lower end of this range, close to the propane boiling point.

Step-by-Step Solution:

Verification / Alternative check:
Practical LPG handling guidelines reference rapid vaporisation near −40°C for propane-dominant mixes at 1 atm; butane-rich blends would shift the curve upward, yet −40°C remains the common benchmark answer in standard MCQs.

Why Other Options Are Wrong:

• 2 or 30 or 55: Too warm for near-complete vaporisation of propane-rich LPG at 1 atm.
• −5: Closer to butane behaviour; not representative for 95% on typical LPG benchmarks.

Common Pitfalls:
Assuming a single exact value for all LPG blends; composition varies, but the benchmark answer aligns with propane-rich behaviour.

Final Answer:
-40