Aamc 11 #32

[chiddler]

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When equal volumes of MgSO4 and NiF2 are mixed, a precipitate forms. Which of the following pieces of information will be the LEAST useful in identifying the identity of the precipitate?

A. Molar solubility of MgF2
*B. Molar solubility of MgSO4
C. Molar solubility of NiSO4
D. Concentration of the NiF2 solution

A and C are useful for sure. Why is B not useful and why is D useful?

thanks.

 

[DrRichand1]

yea the explaination left me stumped as well.

 

[ridethecliche]

When equal volumes of MgSO4 and NiF2 are mixed, a precipitate forms. Which of the following pieces of information will be the LEAST useful in identifying the identity of the precipitate?

A. Molar solubility of MgF2
*B. Molar solubility of MgSO4
C. Molar solubility of NiSO4
D. Concentration of the NiF2 solution

A and C are useful for sure. Why is B not useful and why is D useful?

thanks.

If MgSO4 is in solution, then it's probably not going to precipitate when there are no common ions. So knowing its solubility doesn't tell you anything.

 

[chiddler]

If MgSO4 is in solution, then it's probably not going to precipitate when there are no common ions. So knowing its solubility doesn't tell you anything.

but we're not considering it alone in solution we're considering it with NiF2.

can you explain with a little more detail, please. even agreeing what what you wrote, i'm having difficulty putting it all together.

 

[Morsetlis]

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.

 

[EnginrTheFuture]

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.

This is spot on... just to elaborate a little

MgSO4 is in solution (it is not in precipitate form) and therefore it has not come to it's Ksp. If you add another solution with ions that are not Mg or SO4, MgSO4 cannot and will not precipitate because essentially all you are adding is water... that addition of NiF2 + water is lowering [Mg] and [SO4]. Therefore, knowing the Ksp of MgSO4 is useless since if it is in solution before, it will not precipitate after. However, if you know the Ksp of something that can precipitate upon the addition of these two, knowing the concentration of any of its constituents is useful.

 

[ridethecliche]

but we're not considering it alone in solution we're considering it with NiF2.

can you explain with a little more detail, please. even agreeing what what you wrote, i'm having difficulty putting it all together.

Okay then.

Common ion effect: if there is a common ion between two different chemicals, then it can lead to something precipitating since the ions are competing for the same 'space'. Two different ions don't usually compete for the same space.

When you have ions in solution that aren't competing for space, what CAN happen, is that you end up with a double displacement style reaction when two of the ions in solution react and can make a compound. If this hypothetical compound's molar solubility is really low, then it'll precipitate.

If you know the original thing is already completely soluble for the amount that's in there, knowing the ksp doesn't help since it's not going to magically crash out of solution without anything else changing.

 

[chiddler]

This is spot on... just to elaborate a little

MgSO4 is in solution (it is not in precipitate form) and therefore it has not come to it's Ksp. If you add another solution with ions that are not Mg or SO4, MgSO4 cannot and will not precipitate because essentially all you are adding is water... that addition of NiF2 + water is lowering [Mg] and [SO4]. Therefore, knowing the Ksp of MgSO4 is useless since if it is in solution before, it will not precipitate after. However, if you know the Ksp of something that can precipitate upon the addition of these two, knowing the concentration of any of its constituents is useful.

oh this makes a lot of sense.

Is D useful because of the subsequent common ion effect with the precipitate?

If B was "concentration of MgSO4", it would also be useful information as well, right?

 

[chiddler]

Okay then.

Common ion effect: if there is a common ion between two different chemicals, then it can lead to something precipitating since the ions are competing for the same 'space'. Two different ions don't usually compete for the same space.

When you have ions in solution that aren't competing for space, what CAN happen, is that you end up with a double displacement style reaction when two of the ions in solution react and can make a compound. If this hypothetical compound's molar solubility is really low, then it'll precipitate.

If you know the original thing is already completely soluble for the amount that's in there, knowing the ksp doesn't help since it's not going to magically crash out of solution without anything else changing.

sorry i don't understand the point you're trying to make besides that a lone solution will not spontaneously crashout. but is ok i understood from the post above.