Solubility Equilibria on MCAT?

[Jay2910]

Hello everyone,

I just wanted to know, if you thought it would be a good idea( or a waste of time) to review Ksp problems. I skimmed the BR book and it doesn't go into this kind of equilibria . . but it does have acid/base equilibra.

Right now, I'm thinking I just may? ignore that and look into what compounds are soluble( and which ones are not), and know factors that may make them soluble, Le Chatiler's principle and all . .. am I missing something else?

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[infmous]

higher ksp = higher solubility

def go over. dont lose anything if you do. i think i had a ksp discrete on my test.

 

[MedPR]

Hello everyone,

I just wanted to know, if you thought it would be a good idea( or a waste of time) to review Ksp problems. I skimmed the BR book and it doesn't go into this kind of equilibria . . but it does have acid/base equilibra.

Right now, I'm thinking I just may? ignore that and look into what compounds are soluble( and which ones are not), and know factors that may make them soluble, Le Chatiler's principle and all . .. am I missing something else?

There's a whole section on ksp in TBR Chemistry chapter 3.

higher ksp = higher solubility

def go over. dont lose anything if you do. i think i had a ksp discrete on my test.

higher ksp does not = higher solubility.

 

[411309]

There's a whole section on ksp in TBR Chemistry chapter 3.



higher ksp does not = higher solubility.

higher molar solubility means more soluble. A lot of the time higher ksp means higher solubility but not always.

 

[MedPR]

higher molar solubility means more soluble. A lot of the time higher ksp means higher solubility but not always.

Situations where higher Ksp indicates higher solubility are coincidences.

Saying higher solubility means higher solubility a lot of the time is equivalent to saying higher Keq means faster reaction rate a lot of the time.

Ksp is used to calculate molar solubility, which is the true indicator of solubility.

 

[MT Headed]

'Good idea (or waste of time)?'

Good idea. Learn the tricks to recognize and solve problems like 4X^3=4x10^-6 and it will buy you time to do the other harder problems.

 

[411309]

Situations where higher Ksp indicates higher solubility are coincidences.

Saying higher solubility means higher solubility a lot of the time is equivalent to saying higher Keq means faster reaction rate a lot of the time.

Ksp is used to calculate molar solubility, which is the true indicator of solubility.

That still does not change the fact that a lot of the time higher ksp compounds have higher solubility, lol. I never said it wasn't a coincidence and I did say molar solubility was the predictor. But if for some reason i was guessing because i dint know about molar solubility, then i would go by a higher ksp.

The thing is once you take the actual mcat it's probably going to be a bit different than what you expect. You're going to have to make well thought out guesses sometimes, based on what sounds better. A lot of the passage based problems you don't even need the passage for example. On the mcat you absolutely have to use the passage for most of them, at least thats how mine was.

 

[MedPR]

That still does not change the fact that a lot of the time higher ksp compounds have higher solubility, lol. I never said it wasn't a coincidence and I did say molar solubility was the predictor. But if for some reason i was guessing because i dint know about molar solubility, then i would go by a higher ksp.

The thing is once you take the actual mcat it's probably going to be a bit different than what you expect. You're going to have to make well thought out guesses sometimes, based on what sounds better. A lot of the passage based problems you don't even need the passage for example. On the mcat you absolutely have to use the passage for most of them, at least thats how mine was.

How can you say that a lot of the time the higher ksp compounds have higher solubility? Whether or not that is true depends on the Ksp values the question is asking you to compare.

You can calculate the molar solubility from the Ksp. Just because you would base your guess on a factor and maybe get it right doesn't mean your guessing method works "a lot of the time."

MX(s) <&#8212;-> M+(aq) + X-(aq) Ksp = [M+][X-] Ksp = (x)(x) = x^2
MX2(s) <&#8212;-> M2+(aq) + 2X-(aq) Ksp = [M2+][X-]^2 Ksp = (x)(2x)^2 = 4x^3
MX3(s) <&#8212;-> M2+(aq) + 3X-(aq) Ksp = [M2+][X-]^3Ksp = (x)(3x)3 = 27x^4
M2X(s) <&#8212;-> 2M+(aq) + X2-(aq) Ksp = [M+]^2[X2-] Ksp = (2x)2(x) = 4x^3
M3X(s) <&#8212;-> 3M+(aq) + X3-(aq) Ksp = [M+]^3[X3-] Ksp = (3x)3(x) = 27x^4

For example, which is more soluble?
Ksp CaCO3 = 8.7*10^-9 mol^2 dm^-6
Ksp CaF2 = 4.0 *10^-11 mol^3 dm^-9

For CaCO3 Ksp = x^2
For CaF2 Ksp = 4x^3

CaF2's Ksp is significantly smaller than CaCO3's Ksp, yet it is more than 10x more soluble.

 

[MT Headed]

You guys are focusing so much on molar solubility and then MCAT is far more likely to hit you with how many grams can you dissolve in one liter. Good times!

 

[huanlina]

Yeah, it is such a nice!