Definition · Plain-language
Molar mass
Molar mass is the mass of one mole of a substance, expressed in grams per mole and equal to the sum of the atomic masses in its formula.
The step most authors miss
Doing CRediT right? Don’t stop at the statement.
A CRediT statement credits you inside one paper. The recognition CRediT was built for happens when those roles are tied to you, persistently. Sign in with your ORCID — free — and claim your CRediT contributions on casrai.org, the home of the standard. They become a verified, portable part of your identity, not a line that disappears into one PDF.
Free: claim your contributions, then export a journal-ready CRediT statement, schema.org structured data, JATS XML, CSV or BibTeX — and preview your public profile. A membership publishes that profile publicly and verifies the journals you serve.
Mass per mole
A mole is a fixed number of particles, so the molar mass tells you how much one mole of a substance weighs. Its units are grams per mole (g/mol). Conveniently, the molar mass of an element in grams per mole is numerically equal to that element’s relative atomic mass, the figure shown on the periodic table. Carbon’s relative atomic mass is about 12, so one mole of carbon atoms has a mass of about 12 grams. This neat correspondence is what makes the mole so useful for everyday weighing.
Calculating the molar mass of a compound
To find the molar mass of a compound, add up the molar masses of all the atoms in its chemical formula, multiplying each element by how many of its atoms appear. For water, H₂O, you add two hydrogen atoms (about 1 g/mol each) to one oxygen atom (about 16 g/mol), giving roughly 18 g/mol. For carbon dioxide, CO₂, you add one carbon (about 12) to two oxygens (about 16 each), giving about 44 g/mol. The subscripts in the formula tell you how many of each atom to count.
Why molar mass matters
Molar mass is the practical link between the microscopic world of atoms and the macroscopic world of grams you can weigh on a balance. Chemists cannot count individual atoms, but they can weigh substances, and molar mass converts between a measured mass and a number of moles — and therefore a number of particles. This conversion underpins quantitative chemistry, or stoichiometry: by working in moles, a balanced equation can be used to predict exactly how much product a given mass of reactant will yield.
Key facts
At a glance
- Definition: the mass of one mole of a substance
- Units: grams per mole (g/mol)
- For an element: equals its relative atomic mass in grams
- For a compound: sum of the atomic masses in the formula
- Example: water (H₂O) ≈ 18 g/mol; CO₂ ≈ 44 g/mol
- Why it matters: converts between measurable mass and number of moles
Common misconceptions
What people often get wrong
Often heard: Molar mass and atomic mass are exactly the same thing.
Actually: They are numerically equal for an element but conceptually different. Atomic mass is the mass of a single atom (in atomic mass units); molar mass is the mass of one mole of the substance, in grams per mole.
Often heard: Molar mass is measured in grams.
Actually: Molar mass is measured in grams per mole (g/mol), because it is a mass for each mole of substance. A plain mass in grams refers to a specific sample, not to one mole.
Often heard: You need to know the actual number of atoms to calculate molar mass.
Actually: You only need the chemical formula and the relative atomic masses from the periodic table. Adding the atomic masses for the atoms in the formula gives the molar mass directly.
Going deeper
