Talk:Molar mass

molar mass

Molar mass is the mass of one mole of a chemical element or chemical compound. In SI, the unit is kg/mol. The molar mass can be obtained from the relative molecular mass (still often called erroneously molecular weight and abbreviated by MW) multiplying it by 0.001 kg/mol. there is no element called the dalton is there, it is just a unit of measurement isn't it? —Preceding unsigned comment added by 124.182.147.2 (talk) 07:35, 5 June 2008 (UTC)

"What a tangled web we weave!"

The molecular weight of sugar is 342 and that means there are 342 grams per mole. Its that simple! Why confuse things so tremendously by introducing the obscure Dalton unit and molar mass versus molecular mass?? Wikipedia isn't meant to be read only by PhD physicists and chemists ... it is also meant to be read by us mere mortal masses. - mbeychok 20:50, 25 April 2006 (UTC)

Mass of a mole is called molar mass we can obtained it by molecular mass by miltiplying it by 0.001kg/mol. —Preceding unsigned comment added by 59.144.134.240 (talk • contribs) 12:24, 20 July 2006

The fact is that u, Da, g/mol, kg/mol are all used by physicists, protein mass spectrometrists, chemists, and SI-ists respectively. Tangled yes but we can't really sweep it under the rug. --Rifleman 82 17:06, 5 January 2007 (UTC) as a scientist who has to use Molar concentrations all day long, I think the intro is impossibly confused and complex. It should be something simple like, molar mass is the mass (weight) of one mole of a substance; a mole is 6.02 X 10^23, so the molar mass is the weight of 6.02X10^23 copies of a subtance. For example, the molar mass of 6 X 10^23 atoms of carbon 12 is defined as exctly 12;....] then get more complex as a PhD, I know how hard simplifying things is; the intro sucks (sorry..) howerver, i no longer contribute to wik because *for profits* can re use my work I don't mind if a nonprofit takes what I have done, but I will be dam**ed if I will work for free so some rich scum sucking 1% can get even richer off of my work — Preceding unsigned comment added by 68.236.121.54 (talk) 20:31, 22 June 2012 (UTC)

molar versus molecular mass

Msg. to Nick Y: molar mass is the macroscopic mass of roughly 10²³ molecules, whereas molecular mass is the mass of one molecule. So, they differ in their units when their numeric values are (almost) equal, which is why I deleted usually. Both masses could be expressed in the same units (say kg), but then of course the numeric values would be different by a factor on the order of 10²³. I understand that you know all of this, but for clarity we should be careful about the formulation of these things.--P.wormer 11:36, 11 May 2007 (UTC)

I agree with you for the most part. The issue that I was trying to avoid, which is an idiotic one in my opinion, is that there really is no restriction on units and it is possible to use the same units. I.e. that the molar mass of water could be described as 108.396x10²³ u. Conversely molecular mass may be expressed as some very small number in g/mol. There is no reason to do such a thing but the units do not differ by definition and the insertion of a single word gets around the problem. While we are discussing things I do not see the need to discuss kg/mol in the context of the molar mass versus molecular mass section. I fully understand the need to discuss it in general. It is discussed directly above in the introductory section. If we stick to g/mol within this section for the sake of simplicity of addressing the issue at hand without saying that it must be this or that everything is simpler and clearer. e.g.

"The numeric values of molar mass and molecular mass are approximately equal, although they differ in their units, namely g/mol (chemistry) or kg/mol (SI and physics) for molar mass versus u for molecular mass. For most compounds (when using g/mol and u) the numeric values are not exactly equal but differ slightly."

Could be simply:

"When the molar mass and molecular mass are expressed in g/mol and u respectively they will almost always have similar but not identical numerical values."

--Nick Y. 18:15, 11 May 2007 (UTC)

• Your last sentence is clear and brief. BTW the reason for speaking about kg/mol is that this is an SI unit (not kg/kmol as I thought for a long time). But I too have the habit of ignoring SI, and speak for instance freely in kcal/mol, because I remember those values best. So it is OK by me if we simply use g/mol. Will you make the changes? Thank you, --P.wormer 19:01, 11 May 2007 (UTC)

• While NickY's sentence is clear and brief, unfortunately it is also incorrect. the numerical value of the relative molecular mass and the numerical value of the molar mass can vary by several units. The molar mass of natural water is 18.0152 g/mol, the molecular mass of natural water falls between 18 and 22 amu (not counting tritium isotopers). When you are talking about molecular mass, you simply cannot take only the most abundant isotopomer: it might be simpler, but it's just as simply wrong. Physchim62 (talk) 13:02, 13 October 2007 (UTC)

• I agree completely with you Physchim. My statement is only correct when considering the most abundant isotopers of most elements. The point I was making was that it is common for lay people or even scientists not directly involved in such issues on a regular basis to confuse the molar mass and molecular mass because they are commonly nearly identical numerically. For many common chemicals the most abundant isotopomer will be very close to the isotopic average and using them interchangably will often not have disastrous results and thus such misuse is perpetuated. I personally never ever do this because I measure molecular masses down to several decimal places every day. I am a top candidate for zealtory on this issue but try to moderate and explain. I thank you for the recent improvements they improve the clarity and accuracy. Your work is excellent.--Nick Y. 19:23, 16 October 2007 (UTC)

I'm glad you like them, obviously, and I'm flattered. I still have one or two things I would like to do here, such as a discussion of osmometry as a method of determining molar mass, and may be expand a little more on the question of polymers (not my speciality, but I can try...) The related articles also need to be looked at, especially molecular mass, atomic weight and atomic mass. I don't think there is too much real dispute over these questions so long as we define the terms clearly. Physchim62 (talk) 12:58, 17 October 2007 (UTC)

I have been working slowly on these related articles. They were in horrible shape before I got to them. They said things such as the molecular mass is the same as the molar mass but with different units. I certainly think there is much room for improvement and welcome your working on them. Probably you can find some places where I was not bold enough and too compromising to misnomers. I do think it is important to recognize that it is common practice even amongst chemists to use these terms interchangeably (incorrectly). --Nick Y. 17:14, 17 October 2007 (UTC)

Regarding kg/mol

I'm not sure the emphasis in "kg/mol being the SI unit" is needed, but what worries me the most is that a reader might get the idea that g/mol is not SI. The kilogram may be the base unit, but gram is certainly also SI! An analogous example would be to have an article on intercity distance saying "The SI unit for intercity distance is the meter. However, for historical reasons the kilometer is more commonly used". :-) --Itub 10:47, 17 October 2007 (UTC)

• I agree completely. I have changed the sentence in question. I believe that the odd phrasing of the question was born of of SI unit zealotry and OR through synthesis. kg is base unit of mass therefore kg/mol is base unit of molar mass. Not true. I am glad to have some good editors helping out here.--Nick Y. 17:02, 17 October 2007 (UTC)

regarding the examples

M(H) = 1.007 97(7) × 1 g/mol = 1.007 97(7) g/mol

can someone explain what the 97(7) thing is for dummies? and why the following line is different, only has (5)

Thankyou —Preceding unsigned comment added by 124.176.100.173 (talk) 22:30, 19 January 2009 (UTC)

This is a standard notation for Measurement uncertainty.--Nick Y. (talk) 16:18, 20 January 2009 (UTC)

Gram atomic mass

Gram atomic mass redirects here but this is misleading. A Gram atom is only the same as a mole in the case of monatomic elements. Biscuittin (talk) 16:57, 7 February 2010 (UTC)

I have restored the Gram atomic mass article. Biscuittin (talk) 17:17, 7 February 2010 (UTC)

No, a gram atom is exactly the same as a mole of atoms of an element. You can always have a mole of atoms, whether the elements is monatomic or not. Physchim62 (talk) 21:35, 7 February 2010 (UTC)

Atomic mass or atomic weight

At present we have the definition for an element as "The molar mass of atoms of an element is given by the standard atomic mass of the element multiplied by the molar mass constant". This seems to me to be incorrect for a number of reasons.

1. The dimensions are wrong, since atomic mass is in mass units and the constant has units of mass/mole;
2. The definition for a compound is given as "The molar mass of a compound is given by the sum of the standard atomic weights of the atoms which form the compound multiplied by the molar mass constant" - do we change from atomic mass to atomic weights when considering compounds rather than elements?
3. The reference used to support the definition for an element is Wieser, M. E. (2006), "Atomic Weights of the Elements 2005" (PDF), Pure and Applied Chemistry, 78 (11): 2051–2066, doi:10.1351/pac200678112051 which uses atomic weight, not atomic mass.

I'd suggest that the definition for an element ought to use the atomic weight - does anyone agree? --RexxS (talk) 12:33, 28 May 2010 (UTC)

• • Agreed completely. Atomic masses have relatively little to do with the Molar Mass of an element and definitely not with the relation suggested. The key here is Standard Atomic Weight. --Nick Y. (talk) 17:53, 18 June 2010 (UTC)

Notation needs clarification

In the section describing common Molar masses, parenthesis are used in an uncommon (at least to the layman) manner, resulting in confusion.

M(H) = 1.007 97(7) × 1 g/mol = 1.007 97(7) g/mol

Does the (7) above mean multiplication by 7? Does it mean multiplication by 7th power of 10? Does it mean something else entirely? - JoeOfTheWiki (talk)

It's a concise notation for uncertainty: [1] --RolfSander (talk) 17:16, 13 August 2018 (UTC)

Molar mass and molarity

Is molar mass and molarity is same? What are the symbols used for both. Rupendra singh maura (talk) 20:25, 2 December 2018 (UTC)

Physical quality not-understanding

I just saw molar mass, showing this:

Molar mass
Common symbols	M
SI unit	kg/mol
Other units	g/mol

What a bad performance. -DePiep (talk) 21:36, 16 December 2018 (UTC)

"one mole of that substance"

One what? I've scanned the article and some of the links and I've no idea what "one mole of that substance" might be. A kg? A cup? A swimming-pool? I have no idea. I'm not being deliberately obtuse, and vaguely remember the term "mole" being used when I studied O-level chemistry, in which I managed a rather lacklustre grade C. What I do remember is that a "mole" was never described in any useful context, it appeared one day and we were apparently just expected to accept that it was a thing. I can accept it is a thing, but in order to understand the things that reference it, something a little more specific might be useful! --Vometia (talk) 12:29, 6 May 2019 (UTC)

I have turned mole into a link now.--RolfSander (talk) 12:53, 6 May 2019 (UTC)

formula weight

formula weight redirects here, but isn't discussed here. Specifically, it is important for things that aren't molecular, such as ionic crystals. A standard formula is given, which gives the relative numbers of atoms, even without molecular units. Gah4 (talk) 18:05, 21 June 2019 (UTC)

Examples

Would it be possible to add a few examples for compound calculations? Can be simple, and few examples but just as a quick explanation so that people can check that they did calculate the right values on their own. 2A02:8388:1604:F600:3AD5:47FF:FE18:CC7F (talk) 12:04, 19 July 2021 (UTC)

Molecular weight redirect

Hello, science nerds! Should Molecular weight point here or redirect to Molecular mass?   — TARDIS builder^✉     ★       05:03, 18 June 2024 (UTC)

I think here is correct, because the usual usage of the 3 terms in the chemical literature is inconsistent. Molecular mass and molar mass mean what one expect today: respectively the mass of a single molecule (in g with a value near 10^-27 g) and the mass of a mole (with a value of 1 g or more). However molecular weight is still used as an old-fashioned term normally given in g per mole, so the values are really molar mass although the term "molecular weight" is confusing. This is the practice of most of the chemical literature, so it is not up to Wikipedia to fix. Dirac66 (talk) 02:07, 1 December 2024 (UTC) @TARDIS builder