When considering the mass of a liter of gasoline, it is important to clarify that mass is the measure of matter, while weight is the force exerted by gravity on that mass. Since the mass of gasoline remains constant regardless of location, the scientific discussion centers on its mass, though the terms are often used interchangeably by the public. For liquid gasoline, the mass is not a fixed number but falls within a specific range due to its chemical composition.
The Mass of One Liter of Gasoline
One liter of liquid gasoline typically has a mass between 0.71 and 0.78 kilograms (kg). This range exists because gasoline is not a single compound but a blend of many different hydrocarbons. A commonly used average mass is about 0.74 kilograms per liter, which is significantly less than the 1.0 kilogram mass of one liter of water. This lower mass is why gasoline floats on water. For those using imperial units, this mass translates to approximately 6.0 to 6.5 pounds per US gallon at standard temperature.
Defining Density and Specific Gravity
The reason a liter of gasoline has a mass range instead of a fixed value is explained by the concept of density. Density is defined as mass per unit volume, often expressed as kilograms per liter (\(\text{kg/L}\)). Gasoline is composed of molecules of varying sizes and structures, and the mixture’s exact combination directly determines its density. A higher proportion of heavier, longer-chain hydrocarbon molecules results in a denser, and therefore heavier, liter of fuel.
Specific gravity is a related measurement that provides a simple comparison point for a liquid’s density. It is the ratio of the substance’s density to the density of water at a specified temperature. Gasoline has a specific gravity of roughly 0.70 to 0.77, meaning it is 70% to 77% as dense as water. This unitless number is useful in the petroleum industry for quality control and for converting volumes to mass.
Variables That Change Gasoline Weight
Temperature
Several real-world factors cause the density, and thus the mass, of gasoline to fluctuate. Temperature is a primary variable; as with most liquids, gasoline expands when heated and contracts when cooled. A liter of cold gasoline contains more molecules packed into the volume, making it denser and heavier than a liter of warm gasoline. This change is quantified by the coefficient of thermal expansion, which indicates a volume change of approximately 1% for every \(19^\circ\text{F}\) change in temperature.
Chemical Composition
The specific chemical blend, influenced by the octane rating, also affects the mass. Higher-octane fuels often contain a different ratio of hydrocarbon isomers than regular-grade fuel, resulting in a slight variation in density. Furthermore, the addition of oxygenates like ethanol significantly alters the fuel’s mass, as pure ethanol is denser than pure gasoline. Standard blends like E10 will thus have a slightly greater mass per liter. The refining process also adjusts the blend seasonally, which incidentally affects density.
Practical Consumer Implications
Understanding gasoline’s density has practical implications, particularly for its energy content and the refueling process. Although fuel is sold by volume (liters or gallons), the energy provided to the engine is proportional to the mass of the fuel consumed. This means that a liter of denser, cooler gasoline contains slightly more energy than a liter of warmer, less dense fuel.
The petroleum industry uses temperature compensation to correct for this volume change when fuel is sold in bulk, ensuring distributors receive the correct mass regardless of ambient temperature. While this correction typically does not extend to the consumer pump, the difference for a single fill-up is usually minimal. The slightly increased mass of ethanol-blended fuels is also a factor in energy density, contributing to the slightly lower energy content per volume compared to pure gasoline, which affects a vehicle’s overall fuel economy.