aamc 11 PS#32 solubility

[gentlebeast]

32. Equal volume of MgSO4 and NiF2 mix, and a new precipitate forms. The least helpful information to identify precipitate.

1. solubility of MgSO4
2. concentration of NiF2


I struggled choosing between 1 and 2. It seems to me that both are useless(or useful?). I have difficulty with this concept.

Could someone please explain how to solve this one? Or explain how the solubility and concentration of original reagent have different effects on product? Does "equal volume" play a role here?
I would very much appreciate your help!!

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[SaCkO]

32. Equal volume of MgSO4 and NiF2 mix, and a new precipitate forms. The least helpful information to identify precipitate.

1. solubility of MgSO4
2. concentration of NiF2


I struggled choosing between 1 and 2. It seems to me that both are useless(or useful?). I have difficulty with this concept.

Could someone please explain how to solve this one? Or explain how the solubility and concentration of original reagent have different effects on product? Does "equal volume" play a role here?
I would very much appreciate your help!!

I had the same struggle during the test on this question, and i picked the wrong answer which is concentration of NiF2
it still doesn't make sense to me how aamc picked the right answer..
I understand that if the concentration of NiF2 is high then the probability of one of its ions forming a precipitate is higher due to a higher Q .
But this still does not make a correct answer flash in my head, it is more like a guess and not an educated one for sure

 

[gentlebeast]

I had the same struggle during the test on this question, and i picked the wrong answer which is concentration of NiF2
it still doesn't make sense to me how aamc picked the right answer..
I understand that if the concentration of NiF2 is high then the probability of one of its ions forming a precipitate is higher due to a higher Q .
But this still does not make a correct answer flash in my head, it is more like a guess and not an educated one for sure

This question is hard in a awkward way.
I'm usually confident about PS, but this one does not make sense.

Would you say there are questions like this on real test?

 

[ingramw1202]

32. Equal volume of MgSO4 and NiF2 mix, and a new precipitate forms. The least helpful information to identify precipitate.

1. solubility of MgSO4
2. concentration of NiF2


I struggled choosing between 1 and 2. It seems to me that both are useless(or useful?). I have difficulty with this concept.

Could someone please explain how to solve this one? Or explain how the solubility and concentration of original reagent have different effects on product? Does "equal volume" play a role here?
I would very much appreciate your help!!

You mix magnesium sulfate and NiF2 together and the two dissociate and recombine to form a new precipitate which is either mgF2 or nickel sulfate. The question implies that they dissociate, so the precipitate has to be one of these two. Other wise a "new" precipitate is not formed. Knowing the solubility of magnesium sulfate tells you tells you how much magnesium sulfate dissociates in solution. Knowing the initial concentration of NiF2 will let you know the ratio in which they were combine. The question states that equal volumes of the two were combined. However, we don't know the concentration of either. But we do know that for every one mole of magnesium, two moles of fluorine can combine with it. And for every one mole of nickel, one mole of phosphate will combine with it. If we know concentration, along with some other info about the rate of the reaction between any of the two ions, then you can predict how much precipitate should form for each possible ion combination (theoretical yield) and compare it to the actual yield to determine which precipitate formed. Now if the question gave you the initial concentration of both reactants, than the molar solubility of magnesium sulfate would be great for determining what the precipitate is. But without an initial concentration, it's pretty useless. Remember, they only gace us initial volume.

 

[gentlebeast]

You mix magnesium sulfate and NiF2 together and the two dissociate and recombine to form a new precipitate which is either mgF2 or nickel sulfate. The question implies that they dissociate, so the precipitate has to be one of these two. Other wise a "new" precipitate is not formed. Knowing the solubility of magnesium sulfate tells you tells you how much magnesium sulfate dissociates in solution. Knowing the initial concentration of NiF2 will let you know the ratio in which they were combine. The question states that equal volumes of the two were combined. However, we don't know the concentration of either. But we do know that for every one mole of magnesium, two moles of fluorine can combine with it. And for every one mole of nickel, one mole of phosphate will combine with it. If we know concentration, along with some other info about the rate of the reaction between any of the two ions, then you can predict how much precipitate should form for each possible ion combination (theoretical yield) and compare it to the actual yield to determine which precipitate formed. Now if the question gave you the initial concentration of both reactants, than the molar solubility of magnesium sulfate would be great for determining what the precipitate is. But without an initial concentration, it's pretty useless. Remember, they only gace us initial volume.

Thank you for you insight!!
I think your explanation makes great sense. I never though about comparing actual yield to theoretical yield.

But I think the reason using concentration instead of solubility is that we don't know if MgSO4 and NiF2 solutions are saturated or not. If we know they are saturated, we can still figure out moles using (volume*solubility).

Also, I think this method works only if NiF2 is the limiting reagent. Otherwise, it doesn't help to know the amount of NiF2 because it all depends on MgSO4.

Anyways, thank you for your help! I was lost. At least I gain some clue now.

 

[AirborneBearcat]

You mix magnesium sulfate and NiF2 together and the two dissociate and recombine to form a new precipitate which is either mgF2 or nickel sulfate. The question implies that they dissociate, so the precipitate has to be one of these two. Other wise a "new" precipitate is not formed. Knowing the solubility of magnesium sulfate tells you tells you how much magnesium sulfate dissociates in solution. Knowing the initial concentration of NiF2 will let you know the ratio in which they were combine. The question states that equal volumes of the two were combined. However, we don't know the concentration of either. But we do know that for every one mole of magnesium, two moles of fluorine can combine with it. And for every one mole of nickel, one mole of phosphate will combine with it. If we know concentration, along with some other info about the rate of the reaction between any of the two ions, then you can predict how much precipitate should form for each possible ion combination (theoretical yield) and compare it to the actual yield to determine which precipitate formed. Now if the question gave you the initial concentration of both reactants, than the molar solubility of magnesium sulfate would be great for determining what the precipitate is. But without an initial concentration, it's pretty useless. Remember, they only gace us initial volume.

Thanks, this helped me a lot too. I know this was posted a while ago, but I found this on a google search, as I just took FL #11 and was also very confused about this problem. Decided to post to give this thread a bump up as I'm sure many other people are confused about it, especially since it is the most recent AAMC practice test available.

 

[circulus vitios]

No idea what the hell you guys are talking about theoretical yields for.

MgSO4 is already in solution. And then it forms a non-MgSO4 precipitate. Thus, you do not care about the solubility of something that cannot be the precipitate.