Carbon Monoxide (CO) and Oxygen gas (\(O_2\)) are distinct gases with different physical properties. CO is a compound made of one carbon atom and one oxygen atom, while \(O_2\) is a molecule composed of two oxygen atoms. Determining which gas is “heavier” requires comparing their molecular masses, the scientific measure of a single molecule’s mass. Based on this comparison, Carbon Monoxide is found to be slightly lighter than Oxygen gas, which has implications for how these gases behave in the atmosphere.
The Definition of Molecular Mass
The fundamental property that determines the relative weight of a gas molecule is its molecular mass, also known as molecular weight. This value represents the mass of one molecule of a substance, determined by the specific atoms that make it up. Molecular mass is calculated by summing the atomic masses of all the atoms present in the molecule. The atomic mass for each element is a weighted average that accounts for the natural abundance of that element’s isotopes. Chemists typically measure this mass in grams per mole (g/mol).
Calculating the Mass of Carbon Monoxide and Oxygen
To compare the weights of Carbon Monoxide (CO) and Oxygen gas (\(O_2\)), we use the established atomic masses of their constituent elements. The atomic mass of Carbon (C) is approximately 12.01 g/mol, and the atomic mass of Oxygen (O) is approximately 16.00 g/mol. For Carbon Monoxide (CO), the molecular mass is found by adding the mass of one Carbon atom and one Oxygen atom: 12.01 g/mol + 16.00 g/mol, equaling 28.01 g/mol. Oxygen gas (\(O_2\)) is a diatomic molecule, consisting of two bonded Oxygen atoms. Its molecular mass is calculated by adding the mass of two Oxygen atoms: 16.00 g/mol + 16.00 g/mol, totaling 32.00 g/mol. A direct comparison shows that the molecular mass of Carbon Monoxide (28.01 g/mol) is less than the molecular mass of Oxygen gas (32.00 g/mol).
Density and Real-World Gas Behavior
While molecular mass is a fixed property, a gas’s behavior in the atmosphere is governed by its density, which is a measure of its mass relative to its volume. Gas density ultimately determines whether a gas will tend to rise, sink, or mix with the surrounding air. For any gas, its density is compared to the average density of the air itself.
The average molecular mass of dry air is approximately 28.97 g/mol, as air is a mixture primarily composed of nitrogen (N2) at 28.01 g/mol and oxygen (O2) at 32.00 g/mol. Carbon Monoxide, with a molecular mass of 28.01 g/mol, is extremely close to the average mass of air, making it slightly less dense. This near-equal density is why Carbon Monoxide does not naturally sink or pool close to the floor, correcting a common misconception.
Because the mass of CO is so similar to the average mass of the air, it tends to mix rapidly and thoroughly with the surrounding atmosphere. This fast dispersal is a significant factor in indoor safety, as it means the gas can be found uniformly distributed throughout a room. In contrast, Oxygen gas, with its molecular mass of 32.00 g/mol, is significantly heavier than the average air mass. Pure Oxygen gas is denser than air and would tend to sink slightly if released in a pure form.