AAMC 11 Solubility

[regeneration]

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

1. solubility of MgSO4
2. concentration of NiF2





The correct answer is solubility of MgSO4. Could someone explain how the conc. of NiF2 is useful, and why solubility of MgSO4 isn't?

Thanks!

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[akim6890]

I picked the concentration of NiF2 too but their reasoning is entirely based on the fact that MgSO4 is already in solution so the solubility of MgSO4 is irrelevant. Kaplan says that you need to know the concentration of NiF2 so you can know the concentration of ions in solution but that doesn't really make sense to me in terms of identifying what would precipitate.

 

[regeneration]

I picked the concentration of NiF2 too but their reasoning is entirely based on the fact that MgSO4 is already in solution so the solubility of MgSO4 is irrelevant. Kaplan says that you need to know the concentration of NiF2 so you can know the concentration of ions in solution but that doesn't really make sense to me in terms of identifying what would precipitate.

Right. Say you have 1M NiF2, and that it's fully soluble. Then we know that we have 1mol Ni and 1 mol F2. Both form in a 1:1 ratio with So4 and Mg respectively, so this doesn't give you any clues on proportions of precipitate...

 

[Shjanzey]

Right. Say you have 1M NiF2, and that it's fully soluble. Then we know that we have 1mol Ni and 1 mol F2. Both form in a 1:1 ratio with So4 and Mg respectively, so this doesn't give you any clues on proportions of precipitate...

Wouldn't it be 2 moles of F- ? These are ions dissolved in solution.

Solubility (g/L) = Molar Mass * Molarity <-- I am not sure this applies here, because when you mix both solutions the Molarity will change.

You have MX2 solution. k1 = [M][2X]^2 => 4x^3

If you know the concentration of NiF2 you can just plug in for X and find out it's Ksp.

But in the case of this question lets look at why the solubility of MgSO4 isn't important. That is because when you mix MgSO4 and NiF2 together, what is precipitating is NOT MgSO4. If the new precipitate is not MgSO4, then why would you care what it's solubility is? You already know it did not precipitate before mixing, so that is basically a useless metric for this question. The same applies for NiF2 (So the calculations above would be a waste of time )

What would be more useful is the solubilities of the new compounds that COULD form (e.g. -- MgF2 or NiSO4 ). So basically you have to decide which is least useful. If you know solubility of MgSO4 you can do nothing. If you know the concentration of NiF2 and the solubility of the precipitate you may be able to use that somehow.

 

[regeneration]

You have MX2 solution. k1 = [M][2X]^2 => 4x^3

If you know the concentration of NiF2 you can just plug in for X and find out it's Ksp.

This is it. Cheers.

 

[noobtech]

i got this wrong earlier today too! haha going over it, i read again, the question said "solutions" LOL

 

[regeneration]

Wouldn't it be 2 moles of F- ? These are ions dissolved in solution.

Solubility (g/L) = Molar Mass * Molarity <-- I am not sure this applies here, because when you mix both solutions the Molarity will change.

You have MX2 solution. k1 = [M][2X]^2 => 4x^3

If you know the concentration of NiF2 you can just plug in for X and find out it's Ksp.

Thinking about this again, I don't think your "Solubility (g/L) = Molar Mass * Molarity" equation works. You would need info on the Ksp to solve the molar solubility.

 

[Shjanzey]

Thinking about this again, I don't think your "Solubility (g/L) = Molar Mass * Molarity" equation works. You would need info on the Ksp to solve the molar solubility.

Yes, I know, for this question it won't help you much. That is why I explained later in what I wrote above that it is probably useless, however I wanted to include that bit of information to help dispell some confusion about solubility (Inadvertently I added more confusion -- sorry). In this particular case, Ksp for both NiF2 and MgSO4 is also completely useless information.

 

[regeneration]

What I meant was I don't think "Solubility (g/L) = Molar Mass * Molarity" ever works. Solubility is going to be a function of Ksp, not molar mass & molarity. The units just work out.