Editing Molar mass (section)

=== 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>