Aamc 11 #32

[cloak25]

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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

I know the common ion effect applies here but can't quite wrap my head around this problem. Can someone explain this please? Thanks!

Highlight for answer ---> B

 

[Courtjester8]

Orly? Tell me, what common ion do MgSO4 and NiF2 have when they are put into solution?

cj8

 

[Courtjester8]

I'd approach this problem starting with the knowledge that halide salts are typically extremely soluble.

cj8

EDIT: That was meant to be read differently. That hint hinged on the word 'typically.' So what exceptions are there to halide solubility?

 

[cloak25]

Oops you're right. My bad. No common ions. My mind just went blank on this question.

 

[brownbaglunch]

I did use the common ion effect in my reasoning to solve this problem:

You can cross off A and C right away because those two facts are the most important pieces of information you need, leaving B and D.

B and D are similar. Both tell you how many ions are available initially for the precipitation reactions. We have to find a way to distinguish them.

If the concentration of NiF2 is too low, no precipitate will form. Otherwise, the least soluble precipitate will form. D is a maybe.

If MgSO4 has any solubility at all, then more will dissolve as one of the precipitation reactions proceeds (due to the common ion effect.) This makes the solubility of MgSO4 completely unimportant. Correct answer is B.

 

[AnnJ716]

When I did this one, I used the fact that if you know the solubilities of the products, you can figure them out so I cancelled those two out. I know that the concentration of a reactant can also affect the products so I cancelled out the last option and came up with the right answer.

Is that a completely wrong approach to this problem?

 

[brownbaglunch]

That seems valid too.