Talk:Molar mass

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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)[reply]

"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)[reply]

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 (UTC)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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

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)[reply]