Blatand Disregard of Solubility Rules? AAMC Q

[DrBTS]

I just came across a question I got wrong and I'm amazed at this answer because I've always thought that Pb, Ag, Hg were insoluble except for Nitrates? And Also Hydroxides are Insoluble.

Here's the Question:
Which of the following compounds is most likely to be MORE soluble in equimolar HCl than NaOH?

a.) AgCl

b.) Pb(OH)2

c.) CaF2

d.) HI

I chose C, because of the solubility rules, and then the fact that hydrogen would have lechateliered with HCl...but they say that it's B because of the basic anion formed and the increased lechatelier. Can someone shed some light on this issue? thanks.!!: I know I spelled Blatant wrong btw, I'm on a crappy Dell keyboard in a library...sorry thumbup:

---

Members don't see this ad.

 

[texan2414]

Are the a,b,c,d aqueous? If not then I think the neutralization trumps solubility.

And for the answer choice B = the OH- would be neutralized by H+ so the concentration of OH- would decrease prompting more dissolution of precipitate.

 

[DrBTS]

Is that really true? neutralization "trumps" solubility? Can anyone else chime in with why that may or may not be the correct logic here? Specifically when we know hat Pb should be insoluble?

 

[texan2414]

Also remember that EVERYTHING is soluble to some extent. The degree to which a compound is saturated in its equilibrium with a precipitate and dissolved ions is given by Ksp. So don't automatically assume that if its "insoluble" it must never be soluble.

You will encounter many questions with Ksp and your solubility rules would tell you that those compounds are insoluble when in fact they are marginally soluble.

Even AgCl dissociates into some ions at its Ksp value.

 

[sps27]

Correct me if I am wrong but isn't the question asking which of these 4 compounds more soluble in HCl as opposed to their solubility in NaOH? So in that case if we analyze each of those compounds.....

AgCl with HCl (common ion effect, less soluble) AgCl with NaOH (no common ion effect)
Pb(OH)2 with HCl (no common ion effect) Pb(OH)2 with NaOH (common ion effect)
CaF2 with HCl (no common ion) CaF2 with NaOH (no common ion)
HI with HCl (common ion effect) HI with NaOH (no common ion)

So by looking at all 4, I would say the most obvious answer would be Pb(OH)2 since it has a common ion with NaOH, so it will have more solubility with HCl.

Am I totally misreading this question or making it sound too simple? The reason I neglect CaF2 is because there is no common ion in either case so we really don't know for sure. This question sound so familiar. This must be a TBR question, is it not?

 

[texan2414]

You have to consider (1) common ion effect; AND (2) Le-Chatelier's.

Treatment with an acid (HCl) will neutralize whatever [OH] you have in Pb(OH)2 so that would shift the reaction to the right (more precipitate dissolving)- so even though there is no common ion effect, you still have to consider neutralization. Also I guess you got lucky with the NaOH but what if they were comparing HCl and something else?

You're on the right track to solving the problem, so I don't think you're oversimplifying this. Actually this looks like an aamc question btw.

 

[Temperature101]

I just came across a question I got wrong and I'm amazed at this answer because I've always thought that Pb, Ag, Hg were insoluble except for Nitrates? And Also Hydroxides are Insoluble.

Here's the Question:
Which of the following compounds is most likely to be MORE soluble in equimolar HCl than NaOH?

a.) AgCl

b.) Pb(OH)2

c.) CaF2

d.) HI

I chose C, because of the solubility rules, and then the fact that hydrogen would have lechateliered with HCl...but they say that it's B because of the basic anion formed and the increased lechatelier. Can someone shed some light on this issue? thanks.!!: I know I spelled Blatant wrong btw, I'm on a crappy Dell keyboard in a library...sorry thumbup:

Common ion effect....