For seasons 1-9 see:

  1. I question the use of "question" in question titles (search here)
  2. Help make help questions have more helpful titles (search here)
  3. Houston, we have a "Problem" in question titles (search here)
  4. Buzzwords in question titles - season 4 & 5 - experiments and exercises (search here for experiments)
  5. Season 5 (search here for exercises)
  6. I am asking about chemistry, you know, that's why I put it in the title (search here for chemistry and here for chemical*)
  7. I have a doubt about "doubt" in doubt titles (search title:doubt)
  8. Are you confused by confusing question titles including the word "Confusion"?
  9. Buzzwords Season 7 - Number of Moles (reboot) (this post) (Search: number of moles)


  1. Can we edit out unnecessary "statements of weakness" or buzzwords inside the questions' bodies?
  2. Improper question titles/questions

Here on Chemistry.SE we strive for quality of answers. It has been brought to the attention of this community a number of questions asking about ratios or an amount of material needed for a desired concentration.

I would like to open the discussion as to how to address this. Thoughts?

  • 5
    $\begingroup$ This is IMO a crucial topic and you asked an important question. Seriously, I've seen hundreds of textbooks even of university level using "number of moles", and students [rightfully, as they think] assume it's fine when in reality it's just lame editing. Desperate times call for desperate measures; edit whenever possible, but if user refuses to use proper terminology (happened to me a couple of times) I suggest Close → Off-Topic → Other → I'm voting to close this question as off-topic because moles should be counted on Biology.SE (or something similar along these lines). $\endgroup$
    – andselisk Mod
    Apr 4, 2019 at 21:33
  • 1
    $\begingroup$ The show really jumped the shark after season 2. $\endgroup$
    – jonsca
    Apr 4, 2019 at 22:23
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    $\begingroup$ @andselisk I think we need a canonical question "Why should amount of substance not be called 'number of moles'?" so that we can create links to it. $\endgroup$
    – user7951
    Apr 5, 2019 at 8:25
  • 1
    $\begingroup$ @andselisk As much as I hate to create work for myself, I think that mod intervention is better than closing as off-topic $\endgroup$ Apr 5, 2019 at 9:41
  • $\begingroup$ @andselisk If you'd put that into an answer here, I will definitely vote for its addition into the flag list. Desperate times call for desperate measures. $\endgroup$ Apr 6, 2019 at 6:09
  • $\begingroup$ Thanks for this. On a procedural matter, I'm sure we are way least season 6 by now... $\endgroup$ Apr 6, 2019 at 7:12
  • 1
    $\begingroup$ By way of comparison, the issue with "number of moles" is also mentioned in the meta post about units. $\endgroup$
    – user7951
    Apr 6, 2019 at 8:36
  • $\begingroup$ @Loong I think be cause "number of moles" is so pervasive, there should be some bridge to help people understand. $\endgroup$
    – A.K.
    Apr 7, 2019 at 0:23
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    $\begingroup$ I read the comment by @Martin-マーチン in the linked post, with a criticism of the question "How many meters are there between your house and the nearest bakery?". I completely agree with that criticism, because a meter is not a thing. But I disagree that a "mole" is not a thing when referring to an object, since it refers to a number of things. "How many moles of such and such do you have?" is to me equivalent to "How much (what amount) of such and such do you have (in moles)?" But perhaps being sensitive to this sort of nuance is what distinguishes the real chemists from the nots. $\endgroup$
    – Buck Thorn Mod
    Apr 8, 2019 at 10:02
  • 3
    $\begingroup$ It's like the question "how many eggs do you have" versus "how many dozen eggs do you have". Just divide by 12, no?! $\endgroup$
    – Buck Thorn Mod
    Apr 8, 2019 at 10:03

4 Answers 4


"Amount of substance" is often measured in mass units, e.g. kg or lb, or sometimes even in volume, e.g. mL or fl. oz.

Thus, it is very often necessary to clarify that you are talking about the number of corpuscles in a quantity of substance rather than the mass of the corpuscles or the volume they occupy. "Number of moles" seems like an acceptable way to effect such a clarification.

Evidence for frequent use of mass and volume units in quantifying substances

Let's use water as an example substance.

The Google N-grams Viewer provides insight into how often writers use various phrases across a very, very wide swath of written English.

  • gallons of water is by far a more frequent term than moles of water or pounds of water, comprising around 0.000040% of all 3-grams (i.e. three-word English phrases).

  • pounds of water and moles of water both account for around 0.000005% of all 3-grams. That is they occur about 8-fold less frequently than gallons of water.

If you repeat the search using nitrogen as a substance instead of water, and metric units instead of imperial, the results do change, and moles become more common, but kilograms of nitrogen and liters of nitrogen still occur fairly frequently.

Thus, across a variety of substances and units, mass and volume are widely used to quantify the amounts of substances in written English.

What counts as incorrect

Given their prevalence in written English, it seems counterproductive to me to say that "X gallons of water" is an incorrect phrasing, and that people "have to" say "an amount of water occupying X gallons of volume" instead. If we want to communicate with people, we have to speak the same language as they do.

Avoiding ambiguity

Since people use mass and volume units to quantify the amounts of substance fairly frequently in written English, it will often be necessary for writers to clarify that they don't mean amounts in gallons or in kilograms, but rather in moles. Saying "number of moles" seems like a fine way to make this clarification.

  • 2
    $\begingroup$ When people refer to the mass as amount of substance, then that is wrong, too. I don't think we should accommodate wrong terminology; we should go with what is correct and preferred. $\endgroup$ Apr 8, 2019 at 9:12
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    $\begingroup$ @Martin-マーチン I completely agree with this post (until convinced otherwise of course :)). I see no problem with somewhat unorthodox usage of language provided it is not symptomatic of more serious problems with a post. Also, rather than simply stating "this is wrong", it would help to link to clear explanations rather than obtuse semi-philosophical concerns. Same goes for the OP. I see encouraging the proper use of chemical nomenclature (and english) as an important and noble goal, but to answer questions sometimes intent has to be inferred beyond literal interpretation. $\endgroup$
    – Buck Thorn Mod
    Apr 8, 2019 at 9:53
  • $\begingroup$ @Martin-マーチン Probably 99% of visitors to this site will not understand your objections, and although I think I understand them, it is imho often a problem with attribution of true rather than implied meaning to a set of words. But I stand ready to be corrected since this seems to be another continuing frustration shared by this site's regulars. $\endgroup$
    – Buck Thorn Mod
    Apr 8, 2019 at 9:55
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    $\begingroup$ @NightWriter In the sciences we rely on using unambiguous phrasing and standardised definitions; that is the point of SI, IUPAC, and all the other institutions. Unorthodox language is exactly what causes ambiguity, which may in turn lead to false interpretations, and misconceptions, and problems in communication. It happened, that is why these institutions exist. For that it is not a noble goal, but a necessity to use the correct terminology and notation. If 99% of the visitors do not understand my objections, then this is clearly not the site I have been working on for several years. $\endgroup$ Apr 8, 2019 at 11:32
  • 1
    $\begingroup$ @Martin-マーチン I understand the importance of sticking to definitions. Standardization serves at least two purposes: (1) easing your ability to express yourself and (2) easing others ability to understand you. But I think much (not all obviously) of what we consider standardization in this context amounts to guidelines, not strict rules. It's not the law, it's a set of recommendations, with the goals you point out - clarity. If you have difficulty interpreting a question because somebody writes "number of moles", then it is important. Otherwise I think not. $\endgroup$
    – Buck Thorn Mod
    Apr 8, 2019 at 15:06
  • $\begingroup$ @Martin-マーチン And kudos for being willing to pursue this effort. $\endgroup$
    – Buck Thorn Mod
    Apr 8, 2019 at 15:08
  • $\begingroup$ If such phrasing is "wrong" then most people who communicate in English are "wrong". Dogmatic adherence to specialized definitions is a form of obfuscation, not of enlightenment. $\endgroup$
    – Curt F.
    Apr 8, 2019 at 17:17
  • $\begingroup$ google.com/search?&q=%22amount+of+water%22 Notice how most hits are speaking about the "amount of water" in volumetric units. Is that "wrong"? $\endgroup$
    – Curt F.
    Apr 8, 2019 at 17:17
  • $\begingroup$ books.google.com/ngrams/… I guess adherence to the idea that kilograms or liters don't measure the "amount of substance" would mean that you can't say "X kilograms of water", but would have to say, "an amount of water having a mass of X kilograms". But no one talks that way. $\endgroup$
    – Curt F.
    Apr 8, 2019 at 17:20
  • 1
    $\begingroup$ @NightWriter Most of the times and for my own purposes I strive to use the best language I can and know. You will probably find posts on this network that I have authored, which I would now not express in that way. That is my own evolution and my own business. I am not trying to force any of that on anyone else. The whole meta thread wasn't even my idea, and I am not (nor do I want to) spearheading this effort. What I do do is to make others aware of the incorrect terminology, as many use them without even knowing that it is not correct. $\endgroup$ Apr 8, 2019 at 19:02
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    $\begingroup$ [...] Because of many bad experiences trying to improve our questions and answers, and with the backlash you sometimes face, I oftentimes just leave it as is with the comment being there serving as a reminder. There are plenty of questions and answers that are easier to improve then looking for this phrasing. I am indeed personally very annoyed by this usage, so whenever I encounter it, I'll have to be in an especially good mood to not stop reading and move on. This discussion is wearing me out and there is absolutely no gain in it. I would like to stop now. $\endgroup$ Apr 8, 2019 at 19:08
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    $\begingroup$ @CurtF. I have never said that you should not, or must not use the phrasing X kilograms of water, of X gallons of nitrogen, or even X moles of whatever. That is ridiculous and far off the point. I just tried to make the point that when you refer to a quantity, you should use the proper name for it. However, I am tired of discussing this; this is an equally unrewarding fight as with d-orbital involvement in hybridisation... $\endgroup$ Apr 8, 2019 at 19:08
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    $\begingroup$ By the way a search on this site for "number of moles" gives 925 hits. Amount of substance: 594. Amount of material: 421. $\endgroup$
    – Buck Thorn Mod
    Apr 9, 2019 at 18:33
  • $\begingroup$ I do agree with this answer. I could be at the very extreme of the opinion specrum, but is about what I do when I mass a sample. And from next month one mole is the amount of substance containing a number of particles equal to the numerical value of the Avogadro constant, which has the unit mol−1, and relates the molar mass of an amount of substance to its mass. If we are strict, then we must avoid sentences such as "how much water shall I take" and switch to "which volume of water. ...", if not in daily life, at least here in SE, in the lab, etc., which seems a bit forced to me. $\endgroup$
    – Alchimista
    Apr 10, 2019 at 18:36
  • $\begingroup$ And I would like to add that strictly seeking accuracy can introduce unexpected troubles. Shall we then say that one cannot "weight" in grams? $\endgroup$
    – Alchimista
    Apr 10, 2019 at 18:43

Where do we start from... There are two opposing views; both lie to the far ends of each spectrum and I am about to present a centrist view, so this may either satisfy both sides, or anger both. But I will try, anyway.

First, I will note that despite differences in the approach towards this, I dare say that everybody has the same aim at the end of the day: as a scientific site with quality control, we must strive to always use correct and clear terminology. This is indisputable - I am not up for discussing the (de)merits of such an approach.

The difference, of course, lies in how much you value "correct" over "clear", or vice versa. There is again no question that in a scientific context, the phrase "amount of substance" has a completely unambiguous meaning (it refers to - well - the amount, and not the volume or mass). Furthermore, the phrase "number of moles of water" is not scientifically correct. Therefore, if correctness were the only issue here, I would fully agree with editing all instances of "number of moles of X" to "amount of X".

However, at the same time, we must be cognisant of the role that this website plays. It is not a place for peer-reviewed, academic publication, nor is it a place for only practising chemists to converse with one another. The problem, as has already been brought up (albeit somewhat less amicably), is that the word "amount" is commonly used in a lay context to refer to amount, volume, or mass. Therefore, simply dropping "amount of water" into posts without further clarification is liable to confuse lay readers, or even students who are not familiar with this terminology, which I surmise make up a good proportion of the user base. Insofar as we want to retain the custom of non-academics, we cannot directly go ahead with the action as proposed in the question.

So: in the interests of correctness, I am strongly against leaving "number of moles" in posts unedited or uncommented on. And in the interests of clarity, I am likewise strongly against editing "amount of substance" in silently; in fact, silent editing is more dangerous than leaving things as the status quo. We can grit our teeth and bear with technically incorrect terminology, but confusing students and other visitors is a sure way of alienating them.

If we are to make everybody conform to "amount of substance", then at the same time as editing, we have to be able to educate users who have not been taught the correct terminology – not in an esoteric manner, but rather in an approachable and easily understandable manner. Therefore, there is a need for a canonical Q&A on the matter, the title of which I propose to be simply "What does 'amount of substance' mean?". Such a Q&A should take care to not insist that there is only one correct usage of "amount", but should acknowledge that the scientific and lay usage of "amount" can and does differ.

This is no doubt a challenge, but also an opportunity. As this site allows academic chemists, chemistry students, and lay people to interface, it means that we not only have the knowledge to share, but also the audience to share it with.

I would suggest, therefore, to not go on an editing spree, until such a canonical post has been created. After that, yes, feel free to edit "number of moles X" to "amount of X" – with the caveat that there must, at the very least, be a hyperlink to this canonical post.

  • $\begingroup$ Also, I am very busy nowadays, but would be happy to help write this canonical post, as long as somebody could provide me with appropriate references. Unfortunately, I don't feel that I have the time (or energy) to research it from the ground up. $\endgroup$ Apr 8, 2019 at 20:00
  • $\begingroup$ I am interested in seeing the canonical answer. $\endgroup$
    – Buck Thorn Mod
    Apr 9, 2019 at 12:26
  • 1
    $\begingroup$ In which sense number of moles of water is incorrect given that mole is amount of substance? I really don't understand. I agree that number of meters is rarely ear but length is anyway expressed as number of meters. The same for the amount of substance. $\endgroup$
    – Alchimista
    Apr 10, 2019 at 18:49

I'll provide one reason (opinion based) why editing questions should be done with care: search engines. I think one of your goals is very important - keep answers on the site clear and accurate. But as far as the questions are concerned, I don't think accuracy (in particular) is as important. Rather, your focus should be on "searchability". Here's why: when people search for answers to their questions, they will not necessarily use the terms you deem correct. They may well search for "number of moles" and therefore never hit cleaned-up Q&As (unless you left some of the original incorrect terminology in place with an attached explanation).

You the expert may feel there is an optimal way to write a question, but humans, being creative, will come up with lots of different ways to posit one. This is related to the problem of duplication: there are many questions leading to the same canonical answer, so how to get all questions to "hit" the same answer? Perhaps by linking or aliasing questions.

Off-topic aside

Should turning the tide on "numbers of moles" be a priority for this site? Is it a flagbearer for IUPAC (should it be?)

You could readily build a list of offensive practices commonly encountered on this site (uff, I know many of my posts might easily fall in that list), and I'll provide a couple of examples just from today:

Anthropomorphosizing [Electrons don't take vacations]: What do electrons usually do when lead acid battery is idle?

Ducking, using somewhat ambiguous acronyms, misspelling and not using proper capitalization: explanation of why interphase occurs in polymers

Evapration on stirrers is not scientificlly correct [or, is there ever a good time to evaprate?]: Time for evaporation of acetic acid

Back on topic

The wikipedia page on "amount of substance" does a pretty good job outlining the story:

Amount of substance is a standard-defined quantity that measures the size of an ensemble of particles, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount.

Certainly "chemical amount" and "amount of substance" beat "enplethy" (see further below).

Amount of substance replaces the term "number of moles" which should no longer be used according to IUPAC recommendation. However, confusion can arise due to the everyday usage of the term amount and the term "enplethy" has been suggested for international usage to replace amount of substance.

Mass is proportional to amount of substance by a multiplicative constant. Unless you are using a single particle technique, you are probably deducing this amount of substance by use of that multiplicative constant. Which is why Curt F.s answer, although pushing an unorthodox definition, is very reasonable.

On 20 May 2019, an updated, exact definition of the mole will come into effect.5 It is the numerical value of the Avogadro constant, which has the unit mol−1, and relates the molar mass of an amount of substance to its mass. Therefore, the amount of substance of a sample is calculated as the sample mass divided by the molar mass of the substance.

This one left me scratching my chin:

The mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 12g of the isotope carbon-12.

So this is about a "mole" not being a physically tangible object but an abstract unit. Here the usage of "number of ..." is considered appropriate because elementary entities are tangible. Still the offence seems so minor as to not seem worth prosecuting.

In the meantime reading IUPAC's Green book is probably a good idea.

  • 1
    $\begingroup$ Your example seem to be more due to a lack of command of the English language on the OP's part and can be easily edited. To quote andselisk: "This is IMO a crucial topic and you asked an important question. Seriously, I've seen hundreds of textbooks even of university level using 'number of moles', and students [rightfully, as they think] assume it's fine when in reality it's just lame editing. " $\endgroup$
    – A.K.
    Apr 10, 2019 at 17:25
  • $\begingroup$ @A.K. I strongly disagree. All of those posts contained flaws that suggested other causes than only lack of fluency with English, which is not the question. $\endgroup$
    – Buck Thorn Mod
    Apr 10, 2019 at 18:36

Moving answer part from question to decouple the voting:

Given our drive for quality, I believe that this site should not be complacent in perpetuating a phase that is not correct and therefore recommend:

If you find a post using the phrase:

"number of moles"

it should be edited it to say:

"amount of substance expressed in moles"


"amount of material expressed in moles"


"amount of [substance] expressed in moles"

Also you can add a comment and link this question to help the OP and others become more aware: Is “amount of substance” the same thing as “number of moles”?

and help save Martin - マーチン's sanity.


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