Sciencemadness Discussion Board

What's 2M hcl hydrochloric acid?

overload - 11-10-2011 at 17:50

First of all it's a azeotropic solution. It will only reflux without loosing HC at 18% concentration. The concentration usually used when refluxing is 20% so it levels out to 18% on its own. You can get the concentration of HC to about 40% but this is unstable and HC will evolve out quickly if not stored under the proper conditions. The highest stable concentration is 32% but it's common for people to say that 38% is the highest. The reason I say 32% is the highest is because it is easier to store while 38% is pretty hard to store and looses its potency if left in a hot garage for instance.

I think that the way people describe HCL solutions as 2M or 1M is really annoying. Wouldn't it be easier just to write out the concentration as a percentage like "then 9% hcl was added". I'm trying to find some kind of chart or calculator that can tell me what percentage a 2M solution is. I also cant understand why people say "dilute hcl" why not say the percentage or the ph?

Hydrochloric Acid, 37% is 12M (or 12 Molar)

I found the solution!! sigma aldrich has a calculator on their web site! link Here is what I got out of it.

Your stock solution of Hydrochloric Acid is calculated to be 10.532 M based on a density of 1.2 g/mL, a formula weight of 36.46 g/mol, and a concentration of 32% w/w.

To make a 2 M solution, slowly add 94.948 mL of your stock solution to 125 mL deionized water.

Adjust the final volume of solution to 500 mL with deionized water.


[Edited on 13-10-2011 by overload]

Paddywhacker - 11-10-2011 at 20:04

Quote: Originally posted by overload  
... I think that the way people describe HCL solutions as 2M or 1M is really annoying. Wouldn't it be easier just to write out the concentration as a percentage like "then 9% hcl was added". ...


No. The molecular weight of HCl is roughly 36.5, so 2M HCl contains 73 grams per litre of HCl. What could be simpler?

If you want to prepare 2M HCl then how you go about that will depend on what you start with, but you will always end with 73 grams per litre.

Concentrated hydrochloric acid as purchased from a chemical supplier is commonly referred to as being 11M, or roughly 10M if you don't need precision, so a rough and ready 1M HCl used for, say, pH adjustment, would just be a 10% dilution of the concentrated acid.

overload - 11-10-2011 at 20:46

Quote: Originally posted by Paddywhacker  


The molecular weight of HCl is roughly 36.5, so 2M HCl contains 73 grams per litre of HCl



Or someone could have said it like that. XD now I get it.

SmashGlass - 11-10-2011 at 23:03

Am I just underthinking this but if the con of 37% HCl solution is 12 M
then 1000/12 (ie 1L/12M) = 83.333mL/mol of conc. HCl, made up to 1L.
Will give a 1N solution.
If it is 32%, that is roughly 10M. So 100mL/mol...
For a 2N solution double the amount of acid added to 3/4 of the desired
volume of water, then add the remainder of water to make your desired volume.
If you want to be absolutely accurate titrate it.
Scale appropriately for the amounts you want.
Good luck.

fledarmus - 12-10-2011 at 04:31

Quote: Originally posted by SmashGlass  
Am I just underthinking this but if the con of 37% HCl solution is 12 M
then 1000/12 (ie 1L/12M) = 83.333mL/mol of conc. HCl, made up to 1L.
Will give a 1N solution.
If it is 32%, that is roughly 10M. So 100mL/mol...
For a 2N solution double the amount of acid added to 3/4 of the desired
volume of water, then add the remainder of water to make your desired volume.
If you want to be absolutely accurate titrate it.
Scale appropriately for the amounts you want.
Good luck.


Yep, that's pretty much it.

Even on the bottles of concentrated HCl which say "35-37.5%", you can usually find something on the side where they have actually titrated the lot and will give you a more specific percentage that you can do the calculations from. That's good enough for all but the most precise work.

overload - 12-10-2011 at 07:40

Well you guys know how to explain 2M but what about "dilute hcl" as cycloknight said the second post in the akabori topic started by himself.

Dr.Bob - 12-10-2011 at 07:59

Quote: Originally posted by overload  

I also cant understand why people say "dilute hcl" why not say the percentage or the ph?


The pH of 12M HCl will be nearly 0. The pH of 6M HCl is nearly 0.

The pH of 2M HCl will be nearly 0. The pH of 1M HCl is nearly 0.

The pH of 0.1M HCl will be nearly 0. The pH of 0.01M HCl is nearly 0.

So, do you see why people don't use the pH of an HCl solution to describe it's concentration?

For most purposes, dilute HCl is considered to be 1-2M in the labs I have worked in. But if you are just washing an organic layer to remove traces of bases, then it does not matter as long as the mmoles of HCl are many times the mmoles of what you are extracting. Thus 0.1M HCl may be fine as well.

But it is best to just state the Molarity of HCl used.

fledarmus - 12-10-2011 at 08:06

Quote: Originally posted by Dr.Bob  
Quote: Originally posted by overload  

I also cant understand why people say "dilute hcl" why not say the percentage or the ph?



The pH of 0.1M HCl will be nearly 0. The pH of 0.01M HCl is nearly 0.




Actually, the pH of 0.1 M HCl is about 1, and the pH of 0.01M HCl is about 2. Just being pedantic, I agree wholeheartedly with the point you were making.

In my lab, anything below 1M is dilute HCl, and the only time we actually use that term instead of the actual molarity is when we are adding 1M HCl to some unrecorded volume of water doing an extraction. For reactions, molarity is always recorded.

Endimion17 - 12-10-2011 at 13:03

1 M = 1 mol L<sup>-1</sup>

What could be simpler than that? Of course, we've all got used to mass fractions in the terms of recognizing what's "strong" and what's "weak", but for instant calculations, molar concentrations are the best. What the hell can you do with mass fractions? It requires knowing the density, which depends on the temperature, blah blah blah... Too messy.

[Edited on 12-10-2011 by Endimion17]

overload - 13-10-2011 at 13:12

Ok well here is the problem I have and I really want to understand this. In cycloknites second post in the topic he started about the akabori run he says he extracts the initial toluene extracts with "dilute hcl" to remove the amines from the toluene. Dr. Bob says that as long as the mmols of hcl are much more than the mmols of what your extracting you should be fine yet in a post by another user later on in the thread states that "to potent of hcl was added and this destroys the product". Was this user stating the truth? If so what is the correct concentration to use?

SmashGlass - 17-10-2011 at 13:09

OK. Here we go.
A 1 M HCl solution is dilute enough. The amount of 1 M HCl solution you need to use depends on your scale of reaction.
For example: If you have a 10 mmol of amine (i don't care which one, just pick one) remaining from a reaction, it was in excess or did not react (whatever), adding 50 mL of the 1M HCl solution in the workup for extraction should suffice, because you have added 50 mmol of HCl in a diluted fashion. If you are particularly pedantic one may wash your organic extract again with more 1 M HCl (or split the 50 mL in to two 25 mL batches). If you took 50 mL of 6 M HCl this would be too harsh, as 6 M is considered concentrated.

As a rule of thumb when a paper says "dilute HCl" it usually means 1 M HCl, although it can be less, and it can also be 2 M. It depends on the type of end product you are dealing with and what scale the reaction is.

I hope this helps you get a better picture in your mind to what might be going on in the glassware.

White Yeti - 11-12-2011 at 13:49

I don't see what's the big deal, moles, although irritating to the average beginner are extremely useful for everyone else, especially if you're planning on wasting as little acid as possible. Molar concentrations enable you to calculate the mass of HCl you need by measuring the volume of the acid solution instead. Say you have one mole of CaCO3, (~100g) you would have to measure out 500mL of 2M HCl, 1L of 1M HCl, or 2L of a .5M HCl solution. Percentages are not useful because you'll have to convert to moles eventually if you're planning on saving up on reagents.