Oxygen gas, which we commonly encounter as the molecule \(O_2\), is a form of matter, and all matter possesses physical properties, including mass. This means that a given quantity of oxygen, whether compressed in a tank or spread out in the atmosphere, is subject to the force of gravity, giving it weight.
Oxygen: A Substance with Mass
Oxygen is a chemical element, represented by the symbol O, and is composed of atoms. Each oxygen atom has an atomic mass of approximately 16.00 atomic mass units, which quantifies the amount of matter it contains. In its stable, gaseous form, the oxygen we breathe is diatomic, meaning it exists as a molecule with two oxygen atoms chemically bonded together (\(O_2\)). This molecular structure gives the oxygen molecule a molecular mass of about 32.00 atomic mass units. Mass is an intrinsic property defined as the amount of matter in an object, which remains constant regardless of its location.
Weight Versus Mass: The Role of Gravity
The concept of weight is often confused with mass, but the two are distinct measurements in physics. While mass is the measure of the matter within an object, weight is a measure of the force of gravity acting upon that mass. This relationship is defined by the formula: Weight equals mass multiplied by the acceleration due to gravity.
Since weight is a force dependent on gravity, it changes depending on where the object is located. For instance, the mass of an oxygen tank would remain the same whether it was on Earth or the Moon. However, its weight would be significantly less on the Moon because the lunar gravitational pull is only about one-sixth that of Earth’s.
Why Air Doesn’t Feel Heavy: Buoyancy and Pressure
If oxygen and the rest of the gases in the air have weight, it seems counterintuitive that we do not feel crushed by the atmosphere. This lack of perceived heaviness is explained by the concepts of atmospheric pressure and buoyancy.
Atmospheric Pressure
Atmospheric pressure is the collective force exerted by the entire column of air stretching from the top of the atmosphere down to the Earth’s surface. At sea level, this column of air exerts a force of about 14.7 pounds per square inch on every surface, including our bodies. We do not feel this weight because the air pressure inside our bodies, which includes the air in our lungs, perfectly balances the external atmospheric pressure.
Buoyancy
The other factor is buoyancy, which is an upward force exerted by a fluid that opposes the weight of an immersed object. Since air is a fluid, any object within it experiences an upward buoyant force equal to the weight of the air that object displaces, according to Archimedes’ principle. This buoyant force partially cancels out the downward force of gravity, making objects in the air feel lighter than they would in a vacuum. This upward force is sufficient to mask the weight of the oxygen and other gases that surround us.
Real-World Measurement: Weighing the Invisible
The weight of oxygen is routinely measured in laboratory and industrial settings. One common and direct method to determine the weight of a gas is the “weighing by difference” technique. This involves first weighing a rigid container, such as a sturdy glass flask, after all the air has been removed to create a near-perfect vacuum.
The container is then filled with the gas, such as pure oxygen, and weighed again on a sensitive balance. The difference between the two measurements is the precise weight of the oxygen gas that was introduced. Industrial applications, such as filling medical or welding-grade oxygen cylinders, rely on these precise measurements. Oxygen is compressed into tanks at very high pressures, and the quantity is determined by measuring the weight of the gas added or by relating pressure and temperature readings to known density values.