Help with Molarity (M) and moles (mol)

Factual Questions
1

So… for me, mol (moles) is a quantity, and M is molarity, concentration… Molarity is moles per liter (usually)… so 1 M= 1 mol / L

I’m reading an article, from which I have to design an experiment, and I see the notation mmol. Which I believe is abbreviation of millimoles.

I do not see, in any place in the article, a mention to molarity, or M. I don’t see it, I look for it and I cannot find it.

Since the article is just referencing mmoles, I have to go use the molar weight of the substance to convert the millimoles into grams… which is a lot of it, it turns out. Or at least more than what I need to run my experiment. So I adjust accordingly, although still too much of it (IMHO).

When discussing the paper and my attempts to replicate it, she keeps mentioning that I used too much because I misread the mentions of moles for molarity. I point out that I cannot find a mention to molarity anywhere, but instead they mention quantity, millimoles.

She then mentions 1M = 1000mM or mmol, and also "the molecular weight of A is x, which means for 1M, you need 100g/L. 100mM will then be 1g/L or 1mg/ml.

I’m still confused… explanation please? Because it goes against what I immediately think of when I see moles… that is, when I see moles, I think of pure quantity, not quantity related to concentration (which to me would be molarity)…

Unless, of course, the paper did an extremely poor job of explaining concentration and I’m supposed to magically assume that when they say “it was incubated in 100mmol of X substance”, it means “we used x amounts of X per liter of the substance”.

2

Without the paper, it is difficult to say. I agree with your assessment.

This is also correct, assuming your substance is 100 g/mol.

Journal procedures are notoriously ambiguous. Often there is assumed knowledge.

If this statement is directly from the article, I would assume there is a known volume only because I don’t think you wouldn’t incubate something in a dry solid, and you seem to be implying that it must be a solution. Of course, I have no idea what this paper is about.

3

Huh? a 1 molar solution only contains 100 g/liter of the solute if the solute has a molecular weight of exactly 100. A 1M solution of, for example, Tris will contain 121.14 grams Tris per liter.

Sometimes people get sloppy and use mmol, when they mean millimolar. They should be slapped for this, as 10mmol in a hundred ml of solution works out to a 10M solution, which is probably not at all what is wanted.

4

The quote, from the article, taking out the substances and amounts, is as follows:

There is a bracket in the previous sentence noting that the amount of carrier media (per group) is 1mL.

When I calculated c mmol, converting from moles to grams (using molar weight), it was way, way too much for the amount of liquid I was using (I used 1mL per experiment). I decreased the amount about 100-fold, but it was still too much.

While I don’t disagree that there is way too much, my supervisor insists that it is because I misunderstood the paper. I prefer to think that the paper did a fabulously crappy job of explaining it, rather than I’m the dimbulb. :wink:

5

I want to think this is what happened in the paper. They clearly wrote “mmol”, not mM (millimolar). Yet I’m at fault for not thinking of molarity.

My main beef with this is that my supervisor wrote 1M= 1000 mmol, which goes against what I have ingrained in my mind, from what little I remember of general chemistry.

6

Whoops, I missed that. I agree, it’s sloppy writing. Get used to it. I don’t think I ever ran a literature procedure that didn’t require a lot of tweaking like this, and only experience will teach you when what means what. It also depends a lot on the quality of journal. For example, a procedure from JACS is pretty good, but Tett Lett is complete trash. Of course, it sounds like your doing biology, so you’ll never see those journals.

7

The biochemists I work with frequently do this, use mmol when they really mean mM. e.g. buffers are often referred to as x mmol. Quite confusing - maybe it’s just a lazy contraction that has developed.

It would be a 0.1M solution, surely :slight_smile:

8

I emailed the supervisor, basically saying “OKOK, maybe I should’ve taken for granted the person meant molarity and instead wrote mmmol”

Supervisor: Wait, what article are you talking about? The one by XYZ?

:smack: She never sent XYZ’s journal article!

I emailed back saying that no, I had never gotten that article, instead I got two articles by ABC, from which I based the experiments.

She replied with “Oh, yea, you’re right, it makes no sense otherwise. Here, try this reference…”

:smack::smack::smack:

9

I can understand a molecular biologist doing this, but a biochemist? The shame!:eek:

(633squadron was a chemistry major).