I use this example in my HAZMAT and flammable liquids training courses. To give away the magical answer (LOL), it’s all about the UEL of gasoline, which has a Flash Point of -50F and a vapor pressure of 450 mm Hg or 8.7 psig!
It takes surprisingly little gasoline, roughly 8.5 to 9.5 milliliters (about 0.3 fluid ounces or less than two teaspoons) of liquid gasoline to push a 5-gallon container’s headspace past the Upper Explosive Limit (UEL) of 7.6% at 68°F (20°C), assuming complete volatilization.
Here is the breakdown of the physical chemistry and the equilibrium realities that drive this.
Gasoline is a complex blend of hundreds of hydrocarbons, but for process safety modeling, we can calculate this using the Ideal Gas Law and typical averaged properties for the fuel:
- Container Volume (Vtotal): 5 gallons = 18.93 Liters
- Target UEL (CUEL): 7.6% by volume
- Temperature (T): 68°F (20°C = 293.15 K)
- Pressure (P): 1 atm
- Average Molecular Weight (MW): ~105 g/mol
- Liquid Density (p): ~0.74 g/mL
First, determine the volume of vapor required to reach a 7.6% concentration (i.e., UEL) in the 18.93 L space.
Vvapor =Vtotal X 0.076=18.93 L X 0.076 = 1.439 L
Apply the Ideal Gas Law (PV=nRT) using the gas constant R=0.08206 L atm K-1 mol-1:
Convert the moles of vapor to mass, and then divide by the liquid density to find the volume of the original liquid:
m = n X MW = 0.0598 mol X 105 g/mol = 6.28 g
(Note: Depending on the specific seasonal blend and its exact molecular weight, this value can fluctuate up to roughly 9.5 mL).
Calculating the required volume is only half the equation; vapor pressure governs whether the liquid evaporates fast enough to reach that state at 68°F. Gasoline is highly volatile. At 68°F, its equilibrium vapor concentration in a closed container will naturally equalize somewhere between 25% and 40% by volume—massively over-saturating the headspace and rendering the interior environment too rich to burn.
Because of this rapid evaporation, any “empty” but unpurged gas can sitting with its cap on is almost certainly sitting well above its UEL. The hazard window occurs strictly in the transition states.
If you have ever wondered why the plastic gas containers are limited to 5 gallons, this is why! Imagine how long we would be in the flammable range if we were filling a plastic 330-gallon tote! In the five-gallon container, we literally pass through the flammable range in 1-3 seconds! After this, we are above the UEL, meaning the atm within the container is TOO RICH to ignite.
NOTE: I used Google Gemini Pro for the formulas and to do the math.