Insolubility Question on Berkley Review (Bad Question?)

[johnwandering]

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I'm debating a question which I cannot reason out at ALL...

The question is
"If precipitate forms when NaCl is added to aqueous, what conclusion can we draw?"

And the choices are pretty much, which of the following are in the solution:
AgCl (molar solubility= 1.3x10^-5)
ZnCl2 (M solubility= 3.1x10^-2)
SrCl2 (M solubility= 1.3x10^-1)
PbCl2 (M solubility= 1.2x10^-2)

A.) No Zn
B.) is Ag
C.) Both Sr or Pb
D.) no Sr


The answer is B, that there is Ag in the solution.
Now I can understand that as there are associated Molar solubility values. But there is NO MENTION anywhere in the book of which Molar solubility=highly insoluble... (if all of these are insoluble, Then C is a very valid choice)
Just because something precipitates, it is NOT A REASONABLE conclusion to think that the most insoluble ion is in the solution if more than one kind of ion precipitates.

I thought for a second that perhaps something mentioned in the solubility rules would help. and AgCl is listed, but SO IS PbCl2!!!
Wat The Hell?? Isn't PbCl2 insoluble as well??


Now because the answer choice uses the term "(both/or)" this would make C a valid answer as well.



Now there is only two things that can explain this.
There is a value that I don't know for molar solubility threshold and makes something highly insoluble.
Or is this a bad problem???

 

[SoftwareKevin]

What page/question is this? Could you rewrite the question/answers word for word? "Both Sr *or* Pb" doesn't sound right so maybe something is getting lost in transferring from book to forum? It seems like having either Ag or Zn or Sr or Pb will eventually form precipitate if enough NaCl was added.

If answer C reads "Both Sr *AND* Pb" then that would be incorrect because if precipitate forms we don't know that both are in the solution, just that at least one of them would be in the solution leaving answer B as the only correct choice. In order to know both were in the solution we'd have to add enough NaCl so that both would precipitate and their would have to be a noticeable difference (like color) between the solids to prove that two different solids precipitated. That information isn't given in the problem. All we know is that a solid came out.

I'm debating a question which I cannot reason out at ALL...

The question is
"If precipitate forms when NaCl is added to aqueous, what conclusion can we draw?"

And the choices are pretty much, which of the following are in the solution:
AgCl (molar solubility= 1.3x10^-5)
ZnCl2 (M solubility= 3.1x10^-2)
SrCl2 (M solubility= 1.3x10^-1)
PbCl2 (M solubility= 1.2x10^-2)

A.) No Zn
B.) is Ag
C.) Both Sr or Pb
D.) no Sr


The answer is B, that there is Ag in the solution.
Now I can understand that as there are associated Molar solubility values. But there is NO MENTION anywhere in the book of which Molar solubility=highly insoluble... (if all of these are insoluble, Then C is a very valid choice)
Just because something precipitates, it is NOT A REASONABLE conclusion to think that the most insoluble ion is in the solution if more than one kind of ion precipitates.

I thought for a second that perhaps something mentioned in the solubility rules would help. and AgCl is listed, but SO IS PbCl2!!!
Wat The Hell?? Isn't PbCl2 insoluble as well??


Now because the answer choice uses the term "(both/or)" this would make C a valid answer as well.



Now there is only two things that can explain this.
There is a value that I don't know for molar solubility threshold and makes something highly insoluble.
Or is this a bad problem???

 

[johnwandering]

It is in page 208 of the berkley chem review #1

The question is correct
The answer choice in question says verbatim:
"There is both Sr(2+) or Pb(2+) present in the solution"

 

[UMICHPremed]

When they give you the Ksp values then it is logical to assume that the correct answer can be found by examining the respective molar solubilities. Choice B yields the lowest molar solubility X^2 = ~10^-5.

Btw, I used Berkeley review chem too and they did state that molar solubility is indicative of solubility and Ksp is not. Its half a page in the book in my version. Maybe you have an older version.

 

[BerkReviewTeach]

If precipitate forms when NaCl is added to aqueous, what conclusion can we draw?

AgCl (molar solubility= 1.3x10^-5)
ZnCl2 (M solubility= 3.1x10^-2)
SrCl2 (M solubility= 1.3x10^-1)
PbCl2 (M solubility= 1.2x10^-2)

A.) No Zn
B.) is Ag
C.) Both Sr or Pb
D.) no Sr

The answer is B, that there is Ag in the solution.
Now I can understand that as there are associated Molar solubility values. But there is NO MENTION anywhere in the book of which Molar solubility=highly insoluble... (if all of these are insoluble, Then C is a very valid choice)

I've looked through the book twice and can't find this passage and question. I don't have the latest version, so either this is from a later version of the general chem book or it's from some of their extra handouts? Where is it from, because I'd like to see what the passage says.

If I'm thinking about it correctly, it's like the examples on pages 195 and 196, where they show the flow chart where the least soluble compound (one with the lowest molar solubility) precipitates first. In the question you listed, AgCl will precipitate first, because by definition (having the lowest molar solubility) Ag+ is less soluble with Cl- than Zn2+, Sr2+, or Pb2+.

For this question, you are looking for a best answer. Choices A and D are wrong, because a precipitate can help determine what's in there, not what is not in there. But you knew this. So let's consider how many grams of the salt can dissolve into 100 mL of water and then consider the two remaining choices.

AgCl weighs 143.3 g/mole, so with a molar solubility of 1.3 x 10^-5, about 0.00185 g would dissolve into a L of solution, meaning 0.19 mg is all that would dissolve into 100 mL of water. AgCl would definitely precipitate.

SrCl2 weighs 158.6 g/mole, so with a molar solubility of 1.3 x 10^-1, about 21 g would dissolve into a L of solution, meaning 2.1 g would dissolve into 100 mL of water. It would not precipitate unless you added a whole bunch of excess Cl.

PbCl2 weighs 278.2 g/mole, so with a molar solubility of 1.2 x 10^-2, about 3.4 g would dissolve into a L of solution, meaning 0.34 g would dissolve into 100 mL of water. It would not precipitate unless you added a good amount of Cl.

So while I'll agree that with enough Cl-, choice C could be a true statement, it's not the best answer to the question. Does the passage mention an experiment like the one on page 195? That would help put the question in context.

For reference, the first line in the answer to Example 3.32 states that "the salt with the lowest molar solubility forms a precipitate first." Many times the information from the chapter is found in the sample questions and their solutions. The books are designed so that if all you did were the questions, then you'd get all of the information and strategies.

Just because something precipitates, it is NOT A REASONABLE conclusion to think that the most insoluble ion is in the solution if more than one kind of ion precipitates.

I thought for a second that perhaps something mentioned in the solubility rules would help. and AgCl is listed, but SO IS PbCl2!!!
Wat The Hell?? Isn't PbCl2 insoluble as well??

No, but when the difference between molar solubilities is so large, it is reasonable on this question. The MCAT questions are not exact, and you have to make ajudgement call of what the best answer is on several questions. This question is not about applying solubility rules, because it gives you exact numbers. It's about knowing that 1 x 10^-2 M or less for molar solubility is a rough threshold for something being insoluble. This is explained in the answer explanation for question #64 of the solubility chapter.

 

[BerkReviewTeach]

It is in page 208 of the berkley chem review #1

The question is correct
The answer choice in question says verbatim:
"There is both Sr(2+) or Pb(2+) present in the solution"

You are so right that it's there. I'm illiterate!

After reading the passage and question, I have to agree with you that the passage did not specify the threshold values. That makes the question open to debate for sure. But I don't disagree with their answer choice, because B is a better choice than C, although C is not wrong per say.

 

[johnwandering]

Oh wow,
are you an actual BR course teacher?? (I hope so or that would be a silly name) ***


Thanks for the answer, I appreciate it!!
The books have been serving me well.


I had an extra question though. How representative are the BR passages of the real thing??

 

[SoftwareKevin]

You are so right that it's there. I'm illiterate!

After reading the passage and question, I have to agree with you that the passage did not specify the threshold values. That makes the question open to debate for sure. But I don't disagree with their answer choice, because B is a better choice than C, although C is not wrong per say.

I think it's a typo in the book. Is "Both X or Y" even a grammatically correct thing to say? "Either X or Y" sounds right... "Both X and Y" sounds right... but not "Both X or Y".

I'm going to both sleep or not sleep now
🙂

 

[BerkReviewTeach]

Oh wow,
are you an actual BR course teacher?? (I hope so or that would be a silly name) ***

Thanks for the answer, I appreciate it!!
The books have been serving me well.

I had an extra question though. How representative are the BR passages of the real thing??

At the time I registered here I was a teacher, but now I mostly do odds and ends like editting. If there's ever a case where a teacher gets sick and couldn't teach, I'd fill in.

The BR passages are all over the map. Some are designed to be realistic, some are designed to make you think, some are designed to be weird and make you realize that when you panic, bad things happen, and some are designed to fill your head with the fundamentals. I'd guess that about a third to a half of the passages are representative of what to espect (depending on the chapter). Almost all of them are worth doing for whatever their purpose may be.

I think it's a typo in the book. Is "Both X or Y" even a grammatically correct thing to say? "Either X or Y" sounds right... "Both X and Y" sounds right... but not "Both X or Y".

Grammatically it would seem that it should have read "either X or Y". It's funny with them, because there are suggestions I make when editting that although they agree with, don't get changed because they want to expose students to the poorly worded aspects of some passages. I guess they're right if you look at the AAMC materials that there are some poorly worded sentences and getting ready for the MCAT involves fielding those too. It just grew old as a tutor and teacher always saying "best answer" rather than "correct answer."

Like in the example above, that question has issues, but despite those issues choice B is better than A, C, or D. I get that leaving it this way makes for a better preparation experience, I'm just anal enough that I'd want to change it so it reads better.

 

[SoftwareKevin]

Like in the example above, that question has issues, but despite those issues choice B is better than A, C, or D. I get that leaving it this way makes for a better preparation experience, I'm just anal enough that I'd want to change it so it reads better.

I guess I can concede to that. B is a better answer than C no matter how it's worded 🙂