Acid conc %

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Acid conc %

#1  Postby Stagman » Jan 12, 2013 1:02 am

I remember when I was studying chemistry that one of my teachers mentioned that the maximum concentration of an acid in water is conveniently the same value as its molecular weight. There was no mention of this only being limited to those under 100g/mol, so this may only be the case for a limited ammount of acids.
E.g. H2SO4 has a molecular weight of 98g/mol, and happens to at its max. conc. at 98%
E.g. HCl has a molecular weight of 36g/mol, and happens to at its max. conc. at 36%
Values not exact, but the point is clear. According to various tables I found on the interweb this appears to be a coincidence with a few acids. In any case, I remember there being an actual reason for this and it was explained but I have since long forgotten - despite having a bloody good memory.
Can any of the chemistry wizards here offer an explaination? Or is the above all bollocks at the end of the day?
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Re: Acid conc %

#2  Postby jaydot » Jan 21, 2013 9:59 pm

:popcorn:
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Re: Acid conc %

#3  Postby The Hanging Monkey » Jun 18, 2013 1:01 pm

It's bollocks.
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Re: Acid conc %

#4  Postby DavidMcC » Jun 18, 2013 2:17 pm

By "maximum concentration", presumably you mean maximum H+ ion concentration (ie, maximum strength).
I'm sure the figures you quote, if correct, are illustrative of a coincidence, nothing more.
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Re: Acid conc %

#5  Postby DavidMcC » Jun 18, 2013 2:38 pm

... Although I am not famiiar with the numbers, I am familiar with the fact that acid strength can drop in the absence of sufficient water, because the acid itself becomes covalent instead of ionic (ionic being necessary for acidic strength).
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Re: Acid conc %

#6  Postby natselrox » Jun 18, 2013 4:09 pm

Hydrochloric Acid, 37%, mol wt. 36
Nitric Acid, 70% , mol wt. 63
Phosphoric Acid, 85% , mol wt. 98
Perchloric Acid, 71%, mol wt. 100
Sulfuric Acid, 96%, mol wt. 98

Doesn't really hold.
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Re: Acid conc %

#7  Postby DavidMcC » Jun 18, 2013 5:11 pm

natselrox wrote:Hydrochloric Acid, 37%, mol wt. 36
Nitric Acid, 70% , mol wt. 63
Phosphoric Acid, 85% , mol wt. 98
Perchloric Acid, 71%, mol wt. 100
Sulfuric Acid, 96%, mol wt. 98

Doesn't really hold.

Thanks for the figures, natselrox. The numbers are, indeed well matched for the case mentioned in the OP. That was why I thought it was probably just a coincidence. You have shown that that is, indeed, the case. :thumbup:
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Re: Acid conc %

#8  Postby natselrox » Jun 18, 2013 5:18 pm

It's a good way to remember a few of the common ones though. Maybe there is more to it as well. :smile:
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Re: Acid conc %

#9  Postby DavidMcC » Jun 18, 2013 5:33 pm

natselrox wrote:It's a good way to remember a few of the common ones though. Maybe there is more to it as well. :smile:

But there can't be much in it, because there must be some mineral acids with a molecular weight of more than 100!
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Re: Acid conc %

#10  Postby natselrox » Jun 18, 2013 5:39 pm

DavidMcC wrote:
natselrox wrote:It's a good way to remember a few of the common ones though. Maybe there is more to it as well. :smile:

But there can't be much in it, because there must be some mineral acids with a molecular weight of more than 100!


Oui.

http://en.wikipedia.org/wiki/Chromic_acid (118)
http://en.wikipedia.org/wiki/Hexafluorophosphoric_acid (146)

and others as well.
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Re: Acid conc %

#11  Postby DavidMcC » Jun 18, 2013 5:43 pm

natselrox wrote:
DavidMcC wrote:
natselrox wrote:It's a good way to remember a few of the common ones though. Maybe there is more to it as well. :smile:

But there can't be much in it, because there must be some mineral acids with a molecular weight of more than 100!


Oui.

http://en.wikipedia.org/wiki/Chromic_acid (118)
http://en.wikipedia.org/wiki/Hexafluorophosphoric_acid (146)

and others as well.

:thumbup:
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Re: Acid conc %

#12  Postby trevp » Oct 21, 2013 2:24 pm

Stagman wrote:I remember when I was studying chemistry that one of my teachers mentioned that the maximum concentration of an acid in water is conveniently the same value as its molecular weight. There was no mention of this only being limited to those under 100g/mol, so this may only be the case for a limited ammount of acids.
E.g. H2SO4 has a molecular weight of 98g/mol, and happens to at its max. conc. at 98%
E.g. HCl has a molecular weight of 36g/mol, and happens to at its max. conc. at 36%
Values not exact, but the point is clear. According to various tables I found on the interweb this appears to be a coincidence with a few acids. In any case, I remember there being an actual reason for this and it was explained but I have since long forgotten - despite having a bloody good memory.
Can any of the chemistry wizards here offer an explaination? Or is the above all bollocks at the end of the day?


It's a coincidence. Sulphuric acid can be concentrated to remove all of the water and sulphur trioxide can be dissolved in it to produce an apparent concentration of more than 100% - it's called oleum. However, when concentrating an aqueous solution of sulphuric acid by evaporation (a common technique), it forms a constant boiling mixture (called an azeotrope) at 96% sulphuric acid. Hydrochloric acid is formed by dissolving hydrogen chloride gas in water and the maximuim solubility at atmospheric pressure is 35% by weight (more will dissolve at increased pressure). Similar arguments apply for nitric acid. Although it's only available commercially at 70%, it is possible to produce higher concentrations. It's not related to molecular weight.
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