Molar mass and mass–amount conversion
Molar mass converts a specified substance's amount into mass.
Key knowledge
Molar mass M is mass divided by amount of substance for specified entities and composition, so n = m/M. For entities with mean mass m_entity, M = NA m_entity. Since the 2019 SI revision, NA is exact but the atomic mass constant in kilograms is measured: the molar mass constant Mu = NA mu is close to, and no longer exactly, 0.001 kg/mol.
Values and equations
M = m/n; n = m/M; M = NA m_entity
- M
- molar mass of the specified substance/entities kg mol^-1
- m
- sample mass kg
- n
- amount of the specified entities mol
- NA
- Avogadro constant mol^-1
- m_entity
- mean mass per specified entity kg
Conditions and scope
- Use a consistent entity definition, sample composition and compatible mass units; M must be positive for this conversion.
- A numerical relative atomic mass used as g/mol is an excellent chemical approximation, not an exact post-2019 SI identity.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Inspected SI Brochure printed p.130 equations for M(X), Ar(X), Mu and the post-2019 change, plus Chemistry §3.1 mass–amount calculations. No obsolete exact 12 g/mol carbon-12 molar-mass claim is retained.
2026-10-10 · Independent Mathematics and Information editor (/root/core_math_information). Independent AI editorial review of this entry's statements, equations, applicability conditions and source coverage. Source checks apply to these claims within their stated limits.
Entry details
Permanent link to this entry ↗- Record identity
- foundation-chem-molar-mass
- Revision
- foundations-refresh.1
- Structured reference record
- Foundational reference additions