Talk:Molecular mass

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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)

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[edit] Molar mass in SI

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)


I don't really think this article is a stub, considering the tight focus of the subject. Opinions?

tweekus 02:45, 6 January 2006 (UTC)

I agree. But, well, i could always be wrong.--Solasis 07:25, 16 January 2006 (UTC)

I don't understand the use of an incorrect mass of oxygen for the sample calculation. Why use an erroneous value on this page, even if it is corrected later in the text? What if someone doesn't read that far? I didn't when I came here to look up the value.

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)

I think it's incorrect to say that the mass of a molecule is exactly the sum of the masses of its atoms. That ignores the binding energy - when the molecule was formed, energy was released or absorbed, which (due to the conservation of matter and energy, and E=mc2) slightly affects the mass. Granted the effect is very small and often ignored. 72.34.3.81 01:28, 19 December 2006 (UTC)

[edit] Nomenclature

I fear I am wading into a mess here. A few questions first:

  • I believe the IUPAC should be authoritative. However, molecular mass does not exist in the IUPAC Gold Book. What does exist is relative molecular mass. Should this page be moved?
  • Should molar mass and molecular mass should be merged because they are discussing the same concepts?
  • Does molecular mass refer to the mass (in kg or in u) of one single molecule? Or of one mole? Based on the Mr definition, it does not matter. But when you compare it with molecular weight or molar mass, the difference is very stark.

These is my proposed edit to the lede:

The relative molecular mass, Mr of a chemical compound 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).[1] Since the relative molecular mass is a ratio of masses, it should be a dimensionless number. However, because the magnitudes of molecular mass (sometimes abbreviated M), molecular weight (abbreviated MW), and relative molecular mass are exactly identical, these three terms are often used interchangeably, and relative molecular mass may be seen quoted with the unit g/mol, similar to molecular mass/weight. To be pedantic, weight is not a unit of mass, and the term, molecular weight, is not strictly correct. However, it is commonly used, even more so than Mr or molecular mass due to historic reasons. Most reference texts used in academia and industry use this term.

I feel very strongly that the term molecular weight should not be brushed aside as "former(ly) used" because it is still used in most reference texts. Comments please? --Rifleman 82 17:02, 5 January 2007 (UTC)

I think the articles should be merged, because the different names are used interchangeably in practice. The differences between the terms are subtle and could be dealt with better in a unified article.
Molecular weight is definitely still in use, although it does not correspond to the physical concept of "weight". Something that should be mentioned if the articles are merged is that it is generally not considered correct to use "molecular mass/weight" for substances that are not made of molecules (such as ionic or covalent solids). Itub 19:22, 5 January 2007 (UTC)
This article certainly is a mess! Molar mass and Molecular mass should definitely NOT be merged IMHO, because this will only make people confuse these terms even more than they do already! I don't think the difference in concept is subtle at all, it's like the difference between a molecule and a mole. In practice people confuse them because the numbers are the same (e.g., water is 18 for both), causing the terms to be used interchangably in some informal settings (chatting in the lab!). In designing the chembox, we had to make it clear we were recording molar mass - in the bad old days people often wrote "molecular weight" and "molecular mass" and there were edit wars between the purists and the pragmatics.
I note that Gram-molecular weight is currently a redirect to Mole (unit), whereas molecular weight redirects here. Does anyone object if I change the the Gram-molecular weight redirect so it goes here instead? Walkerma 21:20, 5 January 2007 (UTC)
Go ahead with the redirect change. I agree, the articles are a real mess as well as the overall use of the terminology. Unfortunately some of the IUPAC definitions allow for confusion to persist. Rifleman's changes look positive. I also think we should not merge the articles but should extensively explain the difference in both articles. I am of the strong opinion, despite the IUPAC definition, that relative molar mass and relative molecular mass are not interchangeable. They are after all different numerical values. I think the definitions do not claim them to be interchangeable but simply reflects that they are used interchangeably. I think we should stick to the IUPAC definitions first but also reflect current usage and the errors of current usage. I will be very active in rewriting these articles but want to allow for some consensus building first.--Nick Y. 21:39, 5 January 2007 (UTC)
I believe that relative molecular mass and relative molar mass are almost equivalent, because they are ratios (according to the Gold book definitions) and have the same numerical value. Basically, the relative molecular mass is the mass of the molecule divided by 1/12 of the mass of the carbon-12 atom, while the relative molar mass is the mass of a mole of a substance divided by 1/12 of the mass of a mole of carbon-12. Given that the number of molecules is proportional to the number of moles (which is part of the definition of mole), the two numbers will be absolutely identical and dimensionless. For practical use, yes, people often slap u or g/mol as a unit, but that doesn't make them different concepts IMO, any more than density in g/mL is a different concept than density in kg/L (or in lbs/gal, although in that case the numerical value will be different). I said almost equivalent at the beginning because there is one difference, which is that molecular mass implies the existence of molecules while molar mass doesn't. Itub 22:13, 5 January 2007 (UTC)

I'd agree with Itub that relative molecular mass and relative molar mass are almost equivalent. In fact, I would call them exactly equivalent mathematically. Think about x/y = (exactly equal to) 6.02x / 6.02 y. Nick Y.'s list of definitions seem good. Should we merge everything to do with molecular or molar masses into one single article? Or should we split it into two, molecular mass and molar mass and highlight the close relations between the two? Or should everything have a separate article, all linked by see alsos? I'd prefer merging all into one, because it'll be easy for a user to miss the others if they were split. --Rifleman 82 04:40, 6 January 2007 (UTC)

I don't mean to be disagreeable but I could not disagree more. Molar mass and molecular mass are almost never numerically identical except when addressing average molecular mass. Most molecules have masses that are not equal to the average of the molecules present and similarly not equal to the molar mass. To me molar mass implies the natural abundance of isotopes on earth as found on a standard periodic table whereas average molecular mass implies the local isotopic abundance or something which has been measured. The molar masses are never particularly accurate because the represent the center of a distribution of variable isotopic distributions found around the world often with an emphasis on laboratory settings. I know I am being really nit picking but this is the heart of the problem. Molecular mass is the mass of a molecule, a single molecule of a unique and even improbable isotopic distribution. The average moleular mass is the average mass of a collection of molecules. It is the sum of the molecular masses divided by the number of molecules present. Molar mass is the average of many average molecular masses sampled around the world with error statistics used to determine the number of significant figures. Of course this is actually done with atomic weights and then they are added to compute the molar mass. The number of significant figures today has much more to do with natural variation than with the ability to measure accurately. For example the atomic weight of sodium has many sig figs since it is monoisotopic.--Nick Y. 18:12, 8 January 2007 (UTC)
Ah, thank you Nick, the mass spectrometrist for the clarification. --Rifleman 82 18:16, 8 January 2007 (UTC)
You are right that molecules have different masses due to having different isotopes, etc. The problem is with the use of the terms. I'm sure we can find slightly different usage of these terms among different specialties and among different people, but I think the best option is to base our definitions on authoritative documents such as IUPAC. I think you are taking a usage that is common only in MS as the only usage, while everyone else is happy treating molecular mass/weight and molar mass as synonymous, including IUPAC in the Gold book (taken from the green book, the glossary of physical chemistry terms). Of course, for MS there are more precise terms, listed in the work I cited below, such as monoisotopic mass, exact mass, accurate mass, etc., but these terms should be put in context. I see no problem with saying in this article something like: "in general, molecular mass and molar mass are treated as synonymous. However, in the context of mass spectrometry, molecular mass is used more specifically for [...]" For this reason I still think the articles should be merged, so that the context of each of the dozen or so terms can be discussed in one place. Itub 18:40, 8 January 2007 (UTC)
I thank you for your perspective and I agree that this should not get particularly mass spec centric. I also have no need to change definitions etc. I do however read the IUPAC definitions differently than you and I think this reflect the intentions of the authors. I beleive that IUPAC does intend to imply that molecular mass is the mass of one molecule when they say "a molecule". A single molecule is a single molecule and not an average. I am happy to be corrected and I absolutely think we should reflect all usages, including the incorrect ones. There are just many degrees of correctness and many different contexts. So don't think you are dealing with an over the top mass spectrometry zealot. The common chemistry uses are by far the most important. We should just do the best job that we can to explain all of the subtlties and technicalities in addition to reflecting that for the most part well educated people tend not to care and use them interchangeably.--Nick Y. 18:55, 8 January 2007 (UTC)
Good, then I think we agree more than we disagree. :) This article is a good opportunity to explain all the little-known intricacies behind molecular mass. Itub 19:18, 8 January 2007 (UTC)

[edit] Definitions

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)
Updated. Please jump in if you disagree.--Nick Y. 18:40, 8 January 2007 (UTC)
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)
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)

Updated Again. Please jump in to debate these issues and don't let me go it alone. Other perspectives are very useful and welcome.--Nick Y. 19:39, 16 January 2007 (UTC)

[edit] References

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)

The link is dead now.--Biologos (talk) 07:37, 21 February 2008 (UTC)

[edit] Major Update

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

[edit] Definition

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)

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)

Thanks, Itub. I misunderstood the English: I interpreted "relative to u" as a ratio, such that the unit of molecular mass would be g/u (mass relative to u). I just found "relative to" as "with regard to, in connection with" in the Merriam-Webster. So it was a language problem. Would "expressed in terms of" instead of "relative to" bring the same message across? Might make it even clearer.--Biologos (talk) 07:29, 21 February 2008 (UTC)

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)

The German sister version of this article says, roughly translated: The relative molecular mass is the molecular mass normalized to 1/12 of the mass of carbon 12, and is therefore dimensionless. Would it make sense to change the "relative to" in the English article to "expressed in terms of" or "normalized to"?--Biologos (talk) 10:08, 22 February 2008 (UTC)

[edit] Probable typo in the formula

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)