Molecular mass

The molecular mass (m) is the mass of a given <a href="/facts/Molecule/N9ljSfFq">molecule</a>, often expressed in <a href="/facts/Unit_of_measurement/pvTpWbmC">units</a> of <a href="/facts/Dalton_(unit)/IMATU3vf">daltons</a> (Da). Different molecules of the same compound may have different molecular masses because they contain different <a href="/facts/Isotope/KD9B1y6o">isotopes</a> of an element. The derived quantity relative molecular mass is the <a href="/facts/Unitless/TINrpQIq">unitless</a> ratio of the mass of a molecule to the <a href="/facts/Atomic_mass_constant/IMATU3vf">atomic mass constant</a> (which is equal to one dalton).
The molecular mass and relative molecular mass are distinct from but related to the <a href="/facts/Molar_mass/m2laJErl">molar mass</a>. The molar mass is defined as the mass of a given substance divided by the <a href="/facts/Amount_of_substance/tQVyNLCp">amount of the substance</a>, and is expressed in <a href="/facts/Grams/sRSW5RJb">grams</a> per <a href="/facts/Mole_(unit)/kCMFwCc0">mole</a> (g/mol). That makes the molar mass an average of many particles or molecules (weighted by abundance of the <a href="/facts/Isotope/KD9B1y6o">isotopes</a>), and the molecular mass the mass of one specific particle or molecule. The molar mass is usually the more appropriate quantity when dealing with macroscopic (weigh-able) quantities of a substance.
The definition of molecular weight is most authoritatively synonymous with relative molecular mass, which is dimensionless; however, in common practice, use of this terminology is highly variable. When the molecular weight is given with the unit Da, it is frequently as a weighted average (by abundance) similar to the molar mass but with different units. In <a href="/facts/Molecular_biology/YgqU35ub">molecular biology</a> and <a href="/facts/Biochemistry/fa2pWEBp">biochemistry</a>, the mass of macromolecules is referred to as their molecular weight and is expressed in <a href="/facts/Kilo-/akxTdXyp">kilo</a>daltons (kDa), although the numerical value is often approximate and representative of an average.
The terms "molecular mass", "molecular weight", and "molar mass" may be used interchangeably in less formal contexts where unit- and quantity-correctness is not needed. The molecular mass is more commonly used when referring to the mass of a single or specific well-defined molecule and less commonly than molecular weight when referring to a weighted average of a sample. Prior to the <a href="/facts/2019_revision_of_the_SI/g0h07jCX">2019 revision of the SI</a>, quantities expressed in daltons (Da) were by definition numerically equivalent to molar mass expressed in the units g/mol and were thus strictly numerically interchangeable. After the 2019 revision, this relationship is only approximate, but the equivalence may still be assumed for all practical purposes.
The molecular mass of small to medium size molecules, measured by mass spectrometry, can be used to determine the <a href="/facts/Chemical_composition/vjo4uMQ0">composition</a> of elements in the molecule. The molecular masses of macromolecules, such as proteins, can also be determined by mass spectrometry; however, methods based on <a href="/facts/Viscosity/QLVfrdCz">viscosity</a> and light-scattering are also used to determine molecular mass when <a href="/facts/Crystallographic/9B5toeqG">crystallographic</a> or mass spectrometric data are not available.

Molecular mass open-in-new

Molecular mass