Talk:Molecular mass

message: Although relative molecular mass COULD be considered dimensionless, it is more widely quoted in the units of mass per unit amount (in SI, kg/kmol), where the amount of a substance is given by the total number of molecules divided by Avagadro's constant (approx 6 times ten to the power twenty three).
I move that this be struck from the list of dimensionless numbers on this site, as molecular mass (by it's very name) is not non-dimensional.
:I concur.  It is only with an additional modifier such as relative molecular weight that it could even be considered dimensionless.  Gene Nygaard 02:49, 6 Feb 2005 (UTC)
thread_title: Dimensionless

message: It is commonly beleived that in SI the molar mass of a substance is the mass of one kilomole, therefore it has the unit kg/kmol. This is against the very basic principle of SI. The unit of quantity is mol (defined as containing Avogadro number molecules), the unit of mass is kg, so the unit of molar mass is kg/mol (for instance 0.01802 kg/mol for water.) The molar mass of any substance can be obtained multiplying the relative molecular mass (often still called molecular weight and abbreviated as MW) by 0.001 kg/mol.  
A similar mistake is often made in SI when giving the universal gas constant. Its unit should be J/(mol×K) that is Ru = 8.3145 J/(mol×K) is the correct specification and the use of kmol should be avoided!
It is not erroneus to give Ru = 0.0083145 kJ/(mol×K) though, because any unit can be multiplied by a multiplier kilo and the multiplier can be concatenated with the first term in the numerator. (However, it is difficult to see any advantage of the multiplier in this case.)
--Valko 22:09, 14 December 2005 (UTC)
replies: text: tweekus 02:45, 6 January 2006 (UTC)

text: It's not that one of the masses is incorrect, it's that one of the masses is the mass of a specific isotope, and the other mass is an average of the masses of all the isotopes, weighted by their prevalence.  72.34.3.81 01:28, 19 December 2006 (UTC)

text: Binding energy are O(eV), atomic mass O(GeV).  As well as I know, that makes it within the uncertainty in the determination of atomic mass.  It is, at least, close enough for chemistry.  In quantum mechanics, there is always some uncertainty. Strictly, you should only use exact when it is part of the definition. Gah4 (talk) 23:18, 21 May 2019 (UTC)
thread_title:  Molar mass in SI 

message: I propose that we come up with definitions for all the related terms and agree on what is synonymous with what and how they relate to each other:
Molar Mass:
Molar mass is the mass of one mole of a chemical element or chemical compound. This usually implies the use of standard earth average atomic masses to compute the molar mass.
:*Units: any units of mass with g/mol the most common.
:*Usage: Always used in this meaning
Relative Molar Mass:
The relative molar mass of a substance is the molar mass divided by 1 g mol–1 (one twelfth of the molar mass of pure 12C).
:*Units: unitless (commonly, incorrectly, used with units esp g/mol)
:*Usage: Mainly used in this meaning, although units are often included. Though technically incorrect to include units this is the common practice.
Molecular Mass:
The molecular mass of a substance is the mass of one molecule of that substance. 
:*Units: u or Da
:*Usage: Occasionally misused to mean molar mass, commonly used to mean the relative molecular mass
Relative Molecular Mass:
The relative molecular mass is the ratio of the molecular mass to the unified atomic mass unit.
:*Units: unitless (commonly, incorrectly, used with units esp u or Da)
:*Usage: Infrequently misused to mean molar mass, commonly used with units u or Da. Though technically incorrect to include units this is the common practice.
Mean or Average Molecular Mass:
The mean of the molecular masses of a collection of molecules. (By synthesis, definition not needed, just for clarity)
:*Units: u or Da
:*Usage: Uncommon, mainly used in mass spectrometry where average molecular weight is more common, but since MW is deprecated this would be the correct replacement term.
Molecular Weight:
Officially a deprecated synonym of relative molecular mass. 
:*Units: u or Da 
:*Usage: Very commonly misused to molar mass (where the units g/mol are common).
Monoisotopic mass
The molecular mass of the a single molecule containing only the most common isotope of every element in the molecule.
:*Units: u or Da
:*Usage: Always this meaning. Primarily a mass spectrometry term.
Gram Molecular Mass
The molecular mass in the units of g/mol. Nearly synonymous with molecular mass.
:*Units: g/mol by definition
:*Usage: Always this meaning. Common, but more historical than current.
relative atomic mass (atomic weight)
The ratio of the average mass of the atom to the unified atomic mass unit.
Isotope mass
The mass of a single isotope of an element. (By synthesis, definition not needed, just for clarity)
standard atomic weights
Recommended values of relative atomic masses of the elements revised biennially by the IUPAC Commission on Atomic Weights and Isotopic Abundances and applicable to elements in any normal sample with a high level of confidence. A normal sample is any reasonably possible source of the element or its compounds in commerce for industry and science and has not been subject to significant modification of isotopic composition within a geologically brief period.
Some terms from the MS glossary:
Accurate mass
Experimentally determined mass of an ion that is used to determine an elemental formula.
Exact mass
Calculated mass of an ion or molecule containing a single isotope of each atom. This differs from monoisotopic mass in that it may contain the less abundant isotopes as well as contain a mixture of isotopes; however, only one isotope per atom.
Average mass
Mass of an ion or molecule calculated using the average mass of each element weighted for its natural isotopic abundance.
Nominal mass
Mass of an ion or molecule calculated using the mass of the most abundant isotope of each element rounded to the nearest integer value and equivalent to the sum of the mass numbers of all constituent atoms.
:::I have to run but please edit the above to improve. I know there are some errors primarily in molecular mass since it is often portrayed as a synonym of relative molecular mass.--Nick Y. 22:17, 5 January 2007 (UTC)
replies: text: Updated. Please jump in if you disagree.--Nick Y. 18:40, 8 January 2007 (UTC)
replies: text: I only disagree with saying "misused" or "deprecated", because as I've argued, it depends on the context. Mass spectrometrists may have a very specific definition for a given term, but it doesn't necessarily apply in "general chemistry". Itub 19:02, 8 January 2007 (UTC)
replies: text: By deprecated I mean officially by IUPAC. Misused I am trying to save for technically incorrect by the IUPAC definition. In teh actual article we can choose softer words depending. So for molecular weight we could say something like "Although officially deprecated by IUPAC this is still in very common, almost ubiquitous use." and for misuse we can say things like "although not technically the same according to the strict reading of the IUPAC definitions these terms are used interchangably in most contexts." --Nick Y. 19:33, 8 January 2007 (UTC)
thread_title:  Definitions 

message: A good reference for the mass spectroscopy definitions that don't appear in the Gold book could be this one: STANDARD DEFINITIONS OF TERMS RELATING TO MASS SPECTROMETRY (3rd Draft Document, August 2006). Itub 22:32, 5 January 2007 (UTC)
replies: text: The link is dead now.--Biologos (talk) 07:37, 21 February 2008 (UTC)
thread_title:  References 

message: I have gone ahead and done a major revision. Please give feedback here.--Nick Y. 00:33, 23 January 2007 (UTC)
thread_title: Major Update

message: Hi, right now the first parapraph contains the following two sentences:
The molecular mass [...] is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). This is distinct from the relative molecular mass of a molecule, which is the ratio of the mass of that molecule to 1/12 of the mass of carbon 12 and is a dimensionless number. 
 I'm sorry if I haven't understood the article and the discussion here, but to me "the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12)" is exactly the same as "the ratio of the mass of that molecule to 1/12 of the mass of carbon 12". A mass relative to another mass is a ratio, right?--Biologos (talk) 10:32, 14 February 2008 (UTC)
replies: text: The only difference is that one has units and the other doesn't. The difference only matters to people who worry about things such as the molar mass constant (that is, people who care about "dimensional correctness"). --Itub (talk) 11:09, 18 February 2008 (UTC)

text: I agree it's confusing. Imagine if we had distincts concepts for "metric length" defined as the length of something in meters, and "relative metric length", defined as the dimensionless ratio of the measured length to the meter! ;-) --Itub (talk) 08:03, 21 February 2008 (UTC)
thread_title:  Definition 

message: The formula below, copied from the article, might contain a typo:
molar mass = average molecular mass * (6.022*10−²³g/u)*(6.022*10²³/mol).
Check it for hydrogen (H2).
Not sure why no one fixed this but it's an obvious typo.  I replaced the first 6.022 by 1/6.022 without expanding it to 0.1654 so as to make the math clearer.  --Vaughan Pratt (talk) 07:29, 8 June 2008 (UTC)
thread_title:  Probable typo in the formula 

message: So did we agree on the fact that the molecular mass is the mass of a molecule? Then how could this be?
Molecular mass differs from more conventional measurements by taking into account different isotopic compositions, and as a result Molecular mass is generally more accurate than molar mass, but less used.
Wasn't it the other way around? The definition of the molecular mass on the molar mass page is as follows:
The molecular mass (m) is the mass of a given molecule: it is measured in daltons (Da) or atomic mass units (u), where 1 Da = 1 u = 1.660 538 782(83)×10–27 kg).[3][4] Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The molar mass 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 (weighable) quantities of a substance.  --Ovi 1 (talk) 11:56, 20 July 2008 (UTC)
thread_title: Molecular mass

message: As per the latest IUPAC publication, average hydrogen atomic weight is 1.007975, being its range from 1.00784 to 1.00811, this change some numbers, specially the average molecular weight of water to 18.01535  — Preceding unsigned comment added by 196.216.56.30 (talk) 10:44, 4 June 2012 (UTC)
thread_title:  Unit type variation 

message: I don't think the lead section is too short. It contains a clear definition of the molecular mass and that's all that's needed. Adding more text here will only water down the consise description.--RolfSander (talk) 12:44, 23 March 2013 (UTC)
replies: text: Unless anyone complains now, I will remove the "lead too short" tag again next week.--RolfSander (talk) 13:33, 28 March 2013 (UTC)
replies: text: There were no objections. I have removed the tag again.--RolfSander (talk) 09:59, 3 April 2013 (UTC)
thread_title:  Lead section too short??? 

message: Proper definitions are now given. This is based on the current position in chemistry. 
A large amount has been deleted as useless unreferenced waffle, but editors might take a different view. If so some of it might be reinstated in an historical section, which might also include the old methods based on cryoscopy etc. Petergans (talk) 12:26, 18 October 2013 (UTC)
thread_title:  Major revision 

message: Formula mass currently redirects here. That's a little confusing. We should probably put in a subsection specifically addressing the difference between "formula mass" and "molecular mass" or we should stop redirecting. NickCT (talk) 19:52, 31 October 2013 (UTC)
thread_title:  Formula Mass 

message: (1) The lead section describes the use of mass spectrometry for small to medium molecules. In fact, mass spectrometry has no theoretical upper limit while in practice it is capable of measuring a few hundred kDa (commercial instruments) on up to the 10s-100s of MDa range (e.g., Gary Siuzdak, Mark Bier, Wen-Ping Peng, etc.).
(2) The lead section mentions that mass spectrometry can determine stoichiometry (elemental composition). In practice, this is not strictly true for all m/z measured, as mass measurement inaccuracy increases with increasing m/z.  — Preceding unsigned comment added by Dha250 (talk • contribs) 16:15, 14 November 2013 (UTC)
thread_title:  molecular mass measurement 

message: I just edited the Definitions Section. First it stated that: "Relative atomic and molecular mass values are dimensionless but are given the "unit" Dalton (formerly atomic mass unit) to indicate that the number is equal to the mass of one molecule divided by the mass of one atom of 12C." That is wrong, as can be clearly seen from the definition of Dalton or from the discussions above. It should by divided by 1/12 of the mass of one atom of 12C. I don't know, why noone corrected this obvious mistake. 134.60.31.73 (talk) 11:14, 6 April 2014 (UTC)
thread_title:  Clear Mistake in Definitions Section 

message: formula mass redirects here.  Probably that is fine, but the page should explain the difference between the terms. (The redirect could point to an appropriate section or subsection.)  Also, it seems that formula weight redirects somewhere else.  Seems to me that they should redirect to the same place. Gah4 (talk) 23:24, 21 May 2019 (UTC)
thread_title:  [[formula mass]] 

message: If we have a separate wikipedia page for relative atomic mass and atomic weight redirects to it, then why not create a separate page named relative molecular mass? then, molecular weight may redirect to it, and formula weight can be included as a section of the page. This would clear all the confusion regarding the terms.
Ayushbiswal (talk) 02:38, 13 December 2021 (UTC)
thread_title:  Another Proposed Edit for Relative Molecular Mass and Molar Mass