![]() For example, the relative molecular mass and molecular mass of methane, whose molecular formula is CH 4, are calculated respectively as follows: However, the name unified atomic mass unit (u) is still used in common practice. Relative atomic and molecular masses as defined are dimensionless. For example, water has a relative molecular mass of 18.0153(3), but individual water molecules have molecular masses which range between 18.010 564 6863(15) Da ( 1HĪtomic and molecular masses are usually reported in daltons, which is defined in terms of the mass of the isotope 12C (carbon-12). ![]() The standard atomic weight takes into account the isotopic distribution of the element in a given sample (usually assumed to be "normal"). Molecular masses are calculated from the atomic masses of each nuclide present in the molecule, while relative molecular masses are calculated from the standard atomic weights of each element. The molecular masses of macromolecules, such as proteins, can also be determined by mass spectrometry however, methods based on viscosity and light-scattering are also used to determine molecular mass when crystallographic or mass spectrometric data are not available. The molecular mass of small to medium size molecules, measured by mass spectrometry, can be used to determine the composition of elements in the molecule. After the redefinition of units, this relationship is only nearly equivalent, although the difference is negligible for all practical purposes. Prior to the 2019 redefinition of SI base units 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. ![]() 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. 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. ![]() In molecular biology, the mass of macromolecules is referred to as their molecular weight and is expressed in kDa, although the numerical value is often approximate and representative of an average. When the molecular weight is given with the unit Da, it is frequently as a weighted average similar to the molar mass but with different units. The definition of molecular weight is most authoritatively synonymous with relative molecular mass however, in common practice, use of this terminology is highly variable. The molar mass is usually the more appropriate quantity when dealing with macroscopic (weigh-able) quantities of a substance. That makes the molar mass an average of many particles or molecules, and the molecular mass the mass of one specific particle or molecule. The molar mass is defined as the mass of a given substance divided by the amount of a substance, is expressed in grams per mol (g/mol). The molecular mass and relative molecular mass are distinct from but related to the molar mass. The derived quantity relative molecular mass is the unitless ratio of the mass of a molecule to the atomic mass constant (which is equal to one dalton). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The molecular mass ( m) is the mass of a given molecule.
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