Which principle is used to estimate the LFL of a fuel mixture from the LFLs of its components?

• A. Henry's law
• B. Le Chatelier's principle
• C. Boyle's law
• D. Charles' law

Answer: (B)

The principle used is Le Chatelier's rule for flammability limits. It says that the mixture’s LFL can be estimated from the component LFLs by using a reciprocal-sum relationship: 1/LFL_mix = sum of (fractional amount of each component in the vapor) divided by that component’s LFL. In other words, LFL_mix = 1 / [sum_i (y_i / LFL_i)], where y_i are the volume (or mole) fractions of the components in the mixture.

This captures the idea that each component contributes to the overall flammability of the mixture in proportion to how easily it ignites (its LFL). The more of a component with a low LFL present, the lower the mixture’s overall LFL will be.

Example: if a 50/50 vapor mix of methane (LFL about 5%) and ethane (LFL about 3%) is considered, 1/LFL_mix = 0.5/5 + 0.5/3 = 0.1 + 0.1667 ≈ 0.2667, so LFL_mix ≈ 3.75%. This is an approximation, useful for quick estimates, and assumes ideal mixing without strong interactions or changing conditions.