Aamc mcat cbt 11 ps #32

[dmplz707]

Hey guys so for this question I can't seem to understand why the molar solubility of MgSO4 would be irrelevant. I thought that eq reactions are reversible;

MgSO4 (s) <-> Mg2+ + SO42-

and so wouldn't the molar solubility of MgSO4 be important as well since it still has the possibility of precipitating? Thanks guys.

Item 32

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

A Molar solubility of MgF2

B Molar solubility of MgSO4
The examinee is asked to consider a situation in which two salts are each dissolved in a solvent to make two separate solutions, and then the two solutions are mixed. It is possible for the dissolved ions from each solution to then combine to form a precipitate. The likelihood of precipitate formation for a given cation/anion pair depends on that salt’s molar solubility: the greater the solubility, the greater the tendency for that salt to stay in solution. Alternately, a salt with a low molar solubility will have a greater tendency towards precipitation. Thus, the most important pieces of information are the solubilities of the salts that could form from the ions that encounter one another in the mixed solution. If magnesium cations from the first solution pair up with fluoride anions from the second solution, then a precipitate could form, depending on the molar solubility of that salt. Another combination is possible: the cation from the second solution, nickel(II), could combine to form a precipitate with the anion from the first solution, sulfate. So it would be useful to know the molar solubility of nickel(II) sulfate too. What is NOT needed is the molar solubility of the salts in the original solutions. Because they are in solution already and mixing them with an aqueous solution of a different solute cannot cause precipitate formation, their solubilities are irrelevant. Magnesium sulfate is the salt in the first solution, and knowing its molar solubility will not help answer the question about precipitates that might form once the solutions are mixed. Thus, option B is the best answer.

C Molar solubility of NiSO4

D Concentration of the NiF2 solution

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[MT Headed]

The question stem implies that the MgSO4 solution is not saturated. If you mix it with more water and some other ions, it still wouldn't be saturated, and still would not precipitate. Knowing the solubility of MgSO4 tells us nothing.

Even if the solution was saturated and there were chunks of MgSO4 in the bottom of the beaker, adding more water and other ions wouldn't cause the MgSO4 to precipitate more. In fact, it would probably cause some to dissolve.

You are concerned with the common ion effect, but the fact is there are no ions in common with these two solutions.

 

[DrDotooMuch]

Out of curiosity. Around how many can one miss per section to score around a 12 on each section? Would it be about 1 per passage? Around how many passages per section on the 52 question CBTs. I have the 77 question AAMC test and I'm slightly lost as to how many per passage can be missed for a 12.

Sorry I couldnt help.

 

[salim271]

Out of curiosity. Around how many can one miss per section to score around a 12 on each section? Would it be about 1 per passage? Around how many passages per section on the 52 question CBTs. I have the 77 question AAMC test and I'm slightly lost as to how many per passage can be missed for a 12.

Sorry I couldnt help.

yeah about one per passage, if you get no discretes wrong.

 

[DrDotooMuch]

yeah about one per passage, if you get no discretes wrong.

Awesome. Thanks.

 

[cycloethane]

I'm not entirely clear why the concentration of the NiF2 solution matters. What if there is little of it? What if there is a lot? Why would that matter. The precipitate forms based on molar solubilities... You know that a precipitate forms from the question stem.

 

[cycloethane]

ok ok, I realized why: it is because you need to know what the Q is. Knowing solubilities tells you how likely something is to precipitate. However, this says nothing until you know the concentrations of the chemicals (can be very soluble, but having so few chemicals that no precipitate forms).