solubility clarification

[Oh_Gee]

from GS-9

so what i gleaned from this question is that for 2 ions to precipitate out of solution, their Ksp in solution has to be higher than their reported actual Ksp.

but from TBR GchemI i learned
"Relative solubility questions are asked in many ways such as: "Which salt exhibits greater solvation?" and "Which salt precipitates first?". Use molar solubility to answer questions that address relative solubility. Solubility product is employed only for calculation-based questions."

so you can never use Ksp to determine which salt will precipitate first right?

---

Members don't see this ad.

 

[Czarcasm]

from GS-9

so what i gleaned from this question is that for 2 ions to precipitate out of solution, their Ksp in solution has to be higher than their reported actual Ksp.

but from TBR GchemI i learned
"Relative solubility questions are asked in many ways such as: "Which salt exhibits greater solvation?" and "Which salt precipitates first?". Use molar solubility to answer questions that address relative solubility. Solubility product is employed only for calculation-based questions."

so you can never use Ksp to determine which salt will precipitate first right?

You actually can access the solubility of salts by looking at their Ksp's. You just have to be careful though. As long as you're comparing salts with similar dissociation expressions, then you can treat Ksp as molar solubility (the more accurate comparison of solubility). For instance: MX salts would all have Ksp expressions of x^2. MX2 salts would have Ksp expressions of 4x^3. MX3 would have Ksp expressions of 27x^4. In these types of scenarios, you can reasonably base solubility by just looking at the Ksp, since solving for molar solubility (x) would involve the same step-wise calculation. However, if they differ (ie. MX vs. MX3 salt), then you need to actually solve for molar solubility to make that comparison. Hope this helps.

 

[Teleologist]

For instance: MX salts would all have Ksp expressions of x^2.

Woah, be careful, we need to also be aware that some salts have basic anions which often undergo extensive hydrolysis at the concentrations they are released in solution. Consider silver phosphate; at the initial molarity that phosphate anion is released into solution, extent of hydrolysis is ~100%. Been there done that. Yes I did do the calculation a long time ago.

And yes this is on the MCAT:

hydrolysis of salts of weak acids or bases

https://www.aamc.org/students/download/345244/data/pstopics.pdf

 

[Oh_Gee]

Woah, be careful, we need to also be aware that some salts have basic anions which often undergo extensive hydrolysis at the concentrations they are released in solution. Consider silver phosphate; at the initial molarity that phosphate anion is released into solution, extent of hydrolysis is ~100%. Been there done that. Yes I did do the calculation a long time ago.

And yes this is on the MCAT:



https://www.aamc.org/students/download/345244/data/pstopics.pdf

Ag3PO4>>> Ag+ PO4 ^3-
what do you mean by extensive hydrolysis ?

 

[Teleologist]

What do you think a relatively potent weak base does in water?

 

[Oh_Gee]

What do you think a relatively potent weak base does in water?

very slight dissociation.

i don't understand why you quoted czarism saying
"MX salts would all have Ksp expressions of x^2."

 

[Teleologist]

very slight dissociation.

i don't understand why you quoted czarism saying
"MX salts would all have Ksp expressions of x^2."

I quoted him because he is wrong. So I did not quote for truth.

Also what do you mean very slight dissociation? The phosphate anion falls apart into what now?

 

[Oh_Gee]

I quoted him because he is wrong. So I did not quote for truth.

Also what do you mean very slight dissociation? The phosphate anion falls apart into what now?

but if [MX] >> [M][X]

wouldn't Ksp for a MX always equal x^2




Also what do you mean very slight dissociation?


you asked what do i think happens if weak bases go in water and i said they dissociate slightly

 

[Teleologist]

you asked what do i think happens if weak bases go in water and i said they dissociate slightly

Wait before we clarify the other stuff you need to tell me exactly what happens to weak bases when they come in contact with water.

For example, what happens to the phosphate anion? Can you write me an equation?

 

[Teleologist]

but if [MX] >> [M][X]

wouldn't Ksp for a MX always equal x^2

No, that's incorrect, and I don't know why Czarcasm keeps repeating something that's false.

 

[Oh_Gee]

Wait before we clarify the other stuff you need to tell me exactly what happens to weak bases when they come in contact with water.

For example, what happens to the phosphate anion? Can you write me an equation?

Ag3PO4>>> 3Ag+ + PO4 ^3-

 

[Czarcasm]

Woah, be careful, we need to also be aware that some salts have basic anions which often undergo extensive hydrolysis at the concentrations they are released in solution. Consider silver phosphate; at the initial molarity that phosphate anion is released into solution, extent of hydrolysis is ~100%. Been there done that. Yes I did do the calculation a long time ago.

And yes this is on the MCAT:



https://www.aamc.org/students/download/345244/data/pstopics.pdf

Dude, you're talking nonsense. I was specifically referring to a situation involving the calculation of molar solubility. If you're being asked to compare the solubility of salts based on a table of Ksp's, then it's reasonable to assume they do not dissociate 100%. You over analyze things way too much. I'm not sure if you've taken your MCAT yet, but I promise you that line of thinking will hurt you.

 

[Teleologist]

Dude, you're talking nonsense. I was specifically referring to a situation involving the calculation of molar solubility. If you're being asked to compare the solubility of salts based on a table of Ksp's, then it's reasonable to assume they do not dissociate 100%. You over analyze things way too much.

You misunderstand. I'm not referring to the dissolution of the salt. And again, what is there to assume about the salt dissociating 100%? Of course any semi-soluble salt for which you have a Ksp for probably isn't going to dissociate 100%. I suggest re-reading my statement and re-reading the content outline which states explicitly that what I said is on the MCAT. Now, if you can't understand what I wrote or what the content outline means, here's the place and time to ask me.

I'm not sure if you've taken your MCAT yet, but I promise you that line of thinking will hurt you.

Thanks for the sentiment.

Ag3PO4>>> 3Ag+ + PO4 ^3-

I said specifically the hydrolysis of phosphate anion, not the reaction of the salt with water. In any case the point was that the phosphate anion undergoes extensive hydrolysis with water and as a result calculating s from Ksp alone will yield an incorrect result for molar solubility due to Le Chatlier's principle. This is an example of a salt with a basic anion, and this is on the content outline, so if you want to deny that this is on the MCAT, go ahead. Maybe it won't be on the new MCAT, but as far as I can tell, it is on the current MCAT.

So in the case of silver phosphate Ksp is not reliably estimated from naive Ksp considerations. I would expect most people to set the Ksp of Ag3PO4 = [Ag+]^3[PO4(3-)], which rests upon the faulty assumption that [Ag^3+]=3[PO4(3-)]; there is barely any phosphate anion in the system. A more refined calculation would take into account what the phosphate anion has turned into, and this is "also" Ksp for silver phosphate; see the IUPAC definition of Ksp.

 

[Oh_Gee]

You misunderstand. I'm not referring to the dissolution of the salt. And again, what is there to assume about the salt dissociating 100%? Of course any semi-soluble salt for which you have a Ksp for probably isn't going to dissociate 100%. I suggest re-reading my statement and re-reading the content outline which states explicitly that what I said is on the MCAT. Now, if you can't understand what I wrote or what the content outline means, here's the place and time to ask me.




Thanks for the sentiment.



I said specifically the hydrolysis of phosphate anion, not the reaction of the salt with water. In any case the point was that the phosphate anion undergoes extensive hydrolysis with water and as a result calculating s from Ksp alone will yield an incorrect result for molar solubility due to Le Chatlier's principle. This is an example of a salt with a basic anion, and this is on the content outline, so if you want to deny that this is on the MCAT, go ahead. Maybe it won't be on the new MCAT, but as far as I can tell, it is on the current MCAT.

So in the case of silver phosphate Ksp is not reliably estimated from naive Ksp considerations. I would expect most people to set the Ksp of Ag3PO4 = [Ag+]^3[PO4(3-)], which rests upon the faulty assumption that [Ag^3+]=3[PO4(3-)]; there is barely any phosphate anion in the system. A more refined calculation would take into account what the phosphate anion has turned into, and this is "also" Ksp for silver phosphate; see the IUPAC definition of Ksp.

so we should never assume in a MX >>> M X
that [M]=[X]?

 

[Teleologist]

so we should never assume in a MX >>> M X
that [M]=[X]?

No, first, we don't need to assume anything, and second, no, the only case in which [M]=![X] is when [X] is a significantly basic (or acidic) anion.

 

[Oh_Gee]

No, first, we don't need to assume anything, and second, no, the only case in which [M]=![X] is when [X] is a significantly basic (or acidic) anion.

so would we ever get a question that asks us to calculate [X] and all it gives is a low Ksp and and [MX]

 

[Teleologist]

Doubt it, unless the numbers are really easy. More than likely you'll be asked a conceptual question.

 

[Oh_Gee]

You misunderstand. I'm not referring to the dissolution of the salt. And again, what is there to assume about the salt dissociating 100%? Of course any semi-soluble salt for which you have a Ksp for probably isn't going to dissociate 100%. I suggest re-reading my statement and re-reading the content outline which states explicitly that what I said is on the MCAT. Now, if you can't understand what I wrote or what the content outline means, here's the place and time to ask me.




Thanks for the sentiment.



I said specifically the hydrolysis of phosphate anion, not the reaction of the salt with water. In any case the point was that the phosphate anion undergoes extensive hydrolysis with water and as a result calculating s from Ksp alone will yield an incorrect result for molar solubility due to Le Chatlier's principle. This is an example of a salt with a basic anion, and this is on the content outline, so if you want to deny that this is on the MCAT, go ahead. Maybe it won't be on the new MCAT, but as far as I can tell, it is on the current MCAT.

So in the case of silver phosphate Ksp is not reliably estimated from naive Ksp considerations. I would expect most people to set the Ksp of Ag3PO4 = [Ag+]^3[PO4(3-)], which rests upon the faulty assumption that [Ag^3+]=3[PO4(3-)]; there is barely any phosphate anion in the system. A more refined calculation would take into account what the phosphate anion has turned into, and this is "also" Ksp for silver phosphate; see the IUPAC definition of Ksp.

is this what is meant by 5b?
http://chemwiki.ucdavis.edu/Physical_Chemistry/Equilibria/Solubilty/Hydrolysis#Salt_Hydrolysis

 

[Teleologist]

is this what is meant by 5b?
http://chemwiki.ucdavis.edu/Physical_Chemistry/Equilibria/Solubilty/Hydrolysis#Salt_Hydrolysis

When I see ridiculous stuff like this from that website:

Br- does not hydrolyze; it is an ion.

I tend to lose trust of the author.

In all seriousness, that article is written with the chemical understanding of a 1st grader. I'm glad I didn't go to UC Davis for undergraduate chemistry education.

Just let me know if you have any questions on what I wrote; who better to ask but the man himself?

 

[Oh_Gee]

When I see ridiculous stuff like this from that website:



I tend to lose trust of the author.

In all seriousness, that article is written with the chemical understanding of a 1st grader. I'm glad I didn't go to UC Davis for undergraduate chemistry education.

Just let me know if you have any questions on what I wrote; who better to ask but the man himself?

could you explain that picture you posted? 5 a b and c
what are the key points to know

 

[Teleologist]

5a) I think this is referring to the common ion effect ("added salt").
b) Hydrolysis, or splitting of water, by bases and acids. Obviously bases split water up into hydroxide ions, and acids in general react with water.
c) Self-explanatory.

 

[Oh_Gee]

No, first, we don't need to assume anything, and second, no, the only case in which [M]=![X] is when [X] is a significantly basic (or acidic) anion.

edit: decided to delete this post because you can't have AAMC material without it being posted in title
new thread here
http://forums.studentdoctor.net/threads/aamc-cbt10-q27.1097559/