The Periodic Table

user228700

2:30 AM

In connection to the above discussion about acids and bases, I'd like to ask something. I'm still learning about levelling and differentiating effect of solvents and haven't gotten my head around it yet.

user228700

I understand this effect with regard to water; no acid(/base) stronger than $H_3O^+$(/$OH^-$) can exist in water, simply due to the fact that every acid(/base) ultimately dissociates into $H_3O^+$(/$OH^-$) in water.

user228700

However, I am finding it a little difficult to understand the same effect when it comes to other solvents. And what's this about the possibility of pH being greater than 14/less than 0?

user228700

If you have a good understanding of this concept, please do help me out; I've been struggling to understand this for the past 2 days...

user228700

3:04 AM

I also have a small question about Ostwald's Dilution Law.

user228700

user228700

3:26 AM

After giving this text a read, the question that comes to mind is this; if the concentration of the ions decreases, and the degree of dissociation of the molecule increases at the same time, then shouldn't these two opposing factors balance each other out so that the concentration of the ions remains more or less constant?

user228700

Also, some texts have taken the number of moles of the products and the temperature at which the reaction is held to be constant to derive a relation b/w the volume, $V$, and $K_{eq}$ as $K_{eq}=\sqrtV$. How is this possible, when clearly, the number of moles of the products is not constant, since the dissociation constant changes?

CowperKettle

4:23 AM

Good morning!

I have a table that described the monosaccharide content of a product, and it has entries like "G0-GN,
G0f-GN". I wonder what that means and where to read about it.

user228700

@CowperKettle Morning! :-)

CowperKettle

@KaumudiHarikumar Shubh prabhat!

orthocresol

4:37 AM

@Jan chemistry.stackexchange.com/a/59001/16683 ;)

@CowperKettle no idea :(

CowperKettle

5:18 AM

(0:

Dhanyavaad

user228700

5:31 AM

@CowperKettle :-) Are u trying to learn Hindi?

John Rennie

@KaumudiHarikumar: morning

user228700

@JohnRennie Morning sir :-)

user228700

I kept my promise and didn't even visit the site all Sunday! (:P)

John Rennie

Re your question about levelling - suppose the solvent was ammonia

user228700

@JohnRennie Yes, OK...

John Rennie

5:35 AM

Then replace $\ce{H30+}$ with $\ce{NH4+}$ and the arguments are all the same

Any strong acid in ammonia completely dissociates to give $\ce{NH4+}$

user228700

@JohnRennie Right, OK.

user228700

(Very sorry about that; my mother called me)

John Rennie

As for the Ostwald question, you are quite right that the increased dissociation tends to balance out the dilution to some extent, though of course not completely.

user228700

@JohnRennie OK...

John Rennie

This principle is used in buffer solutions

Well, I suppose the principle isn't completely the same, but it's the same idea.

user228700

5:42 AM

So the increase in dissociation doesn't completely match the increase in dilution?

John Rennie

No

user228700

@JohnRennie Hm, OK. I will get to that later-we have it in the syllabus too.

user228700

@JohnRennie OK. And what about all that about acids having pH less than 0 and bases greater than 14?

John Rennie

The pH is just $-\log_{10}([H^+])$. There is nothing special about the value zero,

user228700

@JohnRennie But at 25°C, the values range from 0 to 14, correct? According to experimental evidence.

John Rennie

5:46 AM

No, you can get $[H^+] > 1$ mol/litre

And if you take log10 of this you get numbers less than zero

But it's certainly true that it's rare to get pH < 0

user228700

@JohnRennie This makes sense but why is it rare..?

John Rennie

Because the common acids have dissociation constants lying in a range where the pH ends up between 0 and 14.

I don't think there's any exciting fundamental principle at work.

user228700

If we've got a strong acid with molarity 2, then $[H^+]=2$ and pH=-2...

user228700

@JohnRennie Right. Never one. Okay.

user228700

So basically, all that the whole discussion about levelling and differentiating solvent tells us is that an acid is only as strong as the acid that the solvent itself produces. And the same goes for bases.

John Rennie

5:51 AM

@KaumudiHarikumar Yes

user228700

I've found that that's one way to think about it. The other way is with regard to the proton accepting(/donating) nature of the solvent...

user228700

ie. If the solvent is a strong base, then even weak acids will have the same strength as strong acids in it because the solvent is strong enough to pull away a proton from both these acids to the sane extent.

user228700

This makes sense, right?

John Rennie

Yes, I think that's a perfectly good way to look at it.

user228700

OK! That's all then. Thanks sir :-)

John Rennie

5:55 AM

Incidentally, I've been Googling for the pH of conc HCl and apparently 10 molar HCL has a pH close to -1.

Though to get -2 would need 100 molar HCl and you'll have problems making a solution that concentrated :-)

user228700

@JohnRennie Oh, I see...

user228700

@JohnRennie Hm, yes.

user228700

59

Q: Is a negative pH level physically possible?

A friend of mine was looking over the definition of pH, and wondering if it is possible to have a negative pH level. From the equation below, it certainly seems possible—just have a $1.1$ (or something $\gt 1$) molar solution of $\ce{H+}$ ions: $$\text{pH} = -\log([\ce{H+}])$$ (Where $[\ce{X}]$ d...

John Rennie

@KaumudiHarikumar aha! It hadn't occurred to me to look on the site.

user228700

6:10 AM

@JohnRennie :-)

user228700

Also sir, do you understand the answer to this question:

user228700

3

Q: What is a strong base?

How come that some insoluble compounds in water, e.g. $\ce{Mg(OH)2}$, $\ce{Ca(OH)2}$, $\ce{Sr(OH)2}$ are considered strong bases? Are they even strong bases? How can I tell if a compound is a strong base or not? Is it possible to explain with equations and/or formulas what happens when these ...