Editing Molar mass (section)

== Related quantities ==

Molar mass is closely related to the '''relative molar mass''' ({{math|''M''{{sub|r}}}}) of a compound and  to the [[Standard atomic weight|standard atomic weights]] of its constituent elements. However, it should be distinguished from the [[molecular mass]] (which is confusingly ''also'' sometimes known as molecular weight), which is the mass of ''one'' molecule (of any ''single'' isotopic composition), and to the [[atomic mass]], which is the mass of ''one'' atom (of any ''single'' isotope). The [[dalton (unit)|dalton]], symbol Da, is also sometimes used as a unit of molar mass, especially in [[biochemistry]], with the definition 1&nbsp;Da&nbsp;= 1&nbsp;g/mol, despite the fact that it is strictly a unit of mass (1&nbsp;Da&nbsp;= 1&nbsp;u&nbsp;= {{val|1.66053906892e−27|(52)|u=kg}}, as of 2022 CODATA recommended values).<ref>{{Cite web |title=CODATA Value: atomic mass constant |url=https://physics.nist.gov/cgi-bin/cuu/Value?u |access-date=2024-06-21 |website=physics.nist.gov}}</ref>

Obsolete terms for molar mass include '''gram atomic mass''' for the mass, in grams, of one mole of atoms of an element, and '''gram molecular mass''' for the mass, in grams, of one mole of molecules of a compound. The '''gram-atom''' is a former term for a mole of atoms, and '''gram-molecule''' for a mole of molecules.<ref name="SI" />

'''Molecular weight''' (M.W.) (for molecular compounds) and '''formula weight''' (F.W.) (for non-molecular compounds), are older terms for what is now more correctly called the '''relative molar mass''' ({{math|''M''{{sub|r}}}}).<ref>{{GoldBookRef|title=relative molar mass|file=R05270}}</ref> This is a [[dimension]]less quantity (i.e., a pure number, without units) equal to the molar mass divided by the [[molar mass constant]].<ref group="notes">The technical definition is that the relative molar mass is the molar mass measured on a scale where the molar mass of unbound [[carbon 12]] atoms, at rest and in their electronic ground state, is 12. The simpler definition given here is equivalent to the full definition because of the way the [[molar mass constant]] is itself defined.</ref>

=== Molecular mass ===
{{main|Molecular mass}}
The molecular mass ({{mvar|m}}) is the mass of a given molecule: it is usually measured in [[Dalton (unit)|dalton]]s (Da or u).<ref name="SI">{{SIbrochure8th|page=126}}</ref> Different molecules of the same compound may have different molecular masses because they contain different [[isotope]]s of an element. This is distinct but related to the molar mass, which is a measure of the average molecular mass of all the molecules in a sample and is usually the more appropriate measure when dealing with macroscopic (weigh-able) quantities of a substance.

Molecular masses are calculated from the [[atomic mass]]es of each [[nuclide]], while molar masses are calculated from the [[standard atomic weight]]s<ref>{{cite web | title = Atomic Weights and Isotopic Compositions for All Elements | url = http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&all=all&ascii=html&isotype=some | publisher = [[NIST]] | access-date = 2007-10-14}}</ref> of each [[Chemical element|element]]. The standard atomic weight takes into account the [[Isotope|isotopic distribution]] of the element in a given sample (usually assumed to be "normal"). For example, [[water (molecule)|water]] has a molar mass of {{val|18.0153|(3)|u=g/mol}}, but individual water molecules have molecular masses which range between {{val|18.0105646863|(15)|u=Da}} ({{chem2|^{1}H2^{16}O}}) and {{val|22.0277364|(9)|u=Da}} ({{chem2|^{2}H2^{18}O}}).

The distinction between molar mass and molecular mass is important because relative molecular masses can be measured directly by [[mass spectrometry]], often to a precision of a few [[Part per million|parts per million]]. This is accurate enough to directly determine the [[chemical formula]] of a molecule.<ref>{{cite web | title = Author Guidelines – Article Layout | url = http://www.rsc.org/Publishing/ReSourCe/AuthorGuidelines/ArticleLayout/sect3.asp | publisher = [[Royal Society of Chemistry|RSC Publishing]] | access-date = 2007-10-14}}</ref>

=== DNA synthesis usage ===
The term '''formula weight''' has a specific meaning when used in the context of DNA synthesis: whereas an individual [[phosphoramidite]] nucleobase to be added to a DNA polymer has protecting groups and has its ''molecular weight'' quoted including these groups, the amount of molecular weight that is ultimately added by this nucleobase to a DNA polymer is referred to as the nucleobase's ''formula weight'' (i.e., the molecular weight of this nucleobase within the DNA polymer, minus protecting groups).{{citation needed|date=August 2022}}