The molar mass of a substance is a fundamental measurement chemists use to relate the mass of a sample to the number of particles it contains. Determining the molar mass of oxygen requires understanding both the mass of a single oxygen atom and the specific form in which oxygen exists under normal conditions. Molar mass determines the mass, in grams, of a standard large quantity of oxygen atoms or molecules.
The Mass of a Single Oxygen Atom
The mass of an individual atom is too small to measure on a standard laboratory scale, so chemists use the specialized unit called the atomic mass unit (\(\text{amu}\)). This unit is defined relative to the carbon-12 atom and allows for a convenient way to express the mass of elements listed on the periodic table. The atomic mass listed for an element represents the weighted average mass of all its naturally occurring isotopes.
For oxygen (chemical symbol O), the standardized atomic mass is approximately 15.999 \(\text{amu}\). This value represents the mass of a single, average oxygen atom.
Why Oxygen is Calculated as \(\text{O}_2\)
The atomic mass value of 15.999 \(\text{amu}\) is for a lone oxygen atom, but oxygen rarely exists in this isolated state in nature. Under typical Earth conditions, oxygen atoms are highly reactive and readily bond with other atoms to achieve a more stable configuration. When oxygen atoms bond with each other, they form a molecule composed of two atoms.
This structure is known as a diatomic molecule, and its chemical formula is \(\text{O}_2\). The oxygen gas found in the atmosphere is almost entirely in this diatomic form. Because the standard state of the element is this two-atom molecule, any calculation of the molar mass of oxygen gas must account for the mass of both atoms in the \(\text{O}_2\) structure.
Calculating the Molar Mass
Molar mass is defined as the mass of one mole of a substance. The concept of the mole is a counting unit, similar to how a dozen counts twelve items, but it represents an incredibly large number of particles. This number, known as Avogadro’s number, is \(6.022 \times 10^{23}\).
The numerical value of an atom’s mass in atomic mass units (\(\text{amu}\)) is equal to the mass of one mole of that atom in grams (\(\text{g/mol}\)). Since the atomic mass of a single oxygen atom is \(15.999 \text{ amu}\), one mole of oxygen atoms (O) has a mass of \(15.999 \text{ g/mol}\). To find the molar mass of the diatomic oxygen molecule (\(\text{O}_2\)), the atomic mass of oxygen must be multiplied by two.
Using the rounded value of \(16.00 \text{ g/mol}\) for a single oxygen atom, the calculation is straightforward. The molar mass of oxygen gas (\(\text{O}_2\)) is determined by adding the mass of the two oxygen atoms together: \(16.00 \text{ g/mol} + 16.00 \text{ g/mol}\). This results in a molar mass of \(32.00 \text{ g/mol}\) for oxygen gas. A sample of oxygen gas weighing \(32.00\) grams contains one mole, or \(6.022 \times 10^{23}\) molecules, of \(\text{O}_2\).