When to do the (x+.001) stuff or just x for molar solubilities.

[510586]

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So I understand that the x goes away because it is too small when you add .1M (or any other value) in some questions such as this one:
"What is the molar solubility of BaF2 in 0.1M NaF (Ksp,BaF2 = 3.2x10-8)", but how come you don't take it into an account when it is asking for concentrations like this question
"What is the maximum concentration of fluoride ions that could be present in 0.032M Ba(NO3)2 (Ksp,BaF2 = 3.2x10-8)". Should you only add the x (even when it is almost always neglible) only when there is a mention of solubility?

 

[FeralisExtremum]

In the first example, you are adding BaF2 to NaF. There is a common ion (F) already in solution, so we must account for its existing concentration. Therefore we use + .1 to indicate the amount of F- already in solution, in addition to the amount that dissolves from BaF2. The resulting expression is [2X + .1]

In the second example: you have BaF2 in Ba(NO3)2, the common ion is Ba and not Fluoride ions, so there is no existing concentration of fluoride in solution (and therefore no [2x + #] needs to be accounted for, it's just [2X]). Try to understand how ICE tables are set up conceptually, it makes understanding this a lot easier. Approaching it like a math problem where you memorize a method of solution and try to apply it the same way every time just ends up making things more confusing.

 

[510586]

Alright thank you I will continue reviewing Chad's videos on the subject to understand it better.

 

[510586]

In the first example, you are adding BaF2 to NaF. There is a common ion (F) already in solution, so we must account for its existing concentration. Therefore we use + .1 to indicate the amount of F- already in solution, in addition to the amount that dissolves from BaF2. The resulting expression is [2X + .1]

In the second example: you have BaF2 in Ba(NO3)2, the common ion is Ba and not Fluoride ions, so there is no existing concentration of fluoride in solution (and therefore no [2x + #] needs to be accounted for, it's just [2X]). Try to understand how ICE tables are set up conceptually, it makes understanding this a lot easier. Approaching it like a math problem where you memorize a method of solution and try to apply it the same way every time just ends up making things more confusing.

sorry actually I meant to ask why we don't add .032 + x to the Ba of the 2nd equation. Isn't there an existing concentration of Ba2+? Kind of like the existing concentration of Fluoride ions?

 

[510586]

Maybe it's because the first example it is dissolving the stuff (molar solubility) so you still need to take into account the x of the ICE chart whereas the second example just asks for concentration that could be present in that given solution.

 

[FeralisExtremum]

sorry actually I meant to ask why we don't add .032 + x to the Ba of the 2nd equation. Isn't there an existing concentration of Ba2+? Kind of like the existing concentration of Fluoride ions?

Ah, you mean like accounting for the common ion effect of the Ba 2+ in solution. The question is asking about F's solubility but I guess the existing Ba in solution would make the BaF2 less soluble, so this problem might be a bit more complicated than I originally thought.

 

[510586]

I feel like I might be overanalyzing it. Just thought it was odd that they did a x+ something in one and not the other. I'm leaning towards the reasoning that one is asking for molar solubilities, and x means molar solubility in ice charts. The other one is asking for concentrations, which you don't need x's for and don't need to put the coefficient in front (like in ice chart), only using the coefficient as an exponent. Like when Chad solves it he just puts [F-] or if it had an exponent, I would assume, [F-]^2 or something.

 

[DAT Destroyer]

So I understand that the x goes away because it is too small when you add .1M (or any other value) in some questions su
ch as this one:
"What is the molar solubility of BaF2 in 0.1M NaF (Ksp,BaF2 = 3.2x10-8)", but how come you don't take it into an account when it is asking for concentrations like this question
"What is the maximum concentration of fluoride ions that could be present in 0.032M Ba(NO3)2 (Ksp,BaF2 = 3.2x10-8)". Should you only add the x (even when it is almost always neglible) only when there is a mention of solubility?

The first question deals with what is known as the Common ion Effect......the salts dealt with are quite insoluble.....adding the 0.1 M NaF is adding quite a bit of Fluoride ions.......as compared to that given off by the salt. The second question you posed is different.....having ZERO to do with a common ion being added.... ....All they want is the small fluoride ion concentration needed to form a saturated solution. I promise you this.....Any Gen Chem text can explain this simply if you need more. I am glad that you do see the slight difference in problem approach......you are correct.

Keep hammering away.

 

[510586]

Thanks orgoman! By the way are you Nancy or Dr. Romano? I feel weird calling you orgoman all the time

 

[510586]

The first question deals with what is known as the Common ion Effect......the salts dealt with are quite insoluble.....adding the 0.1 M NaF is adding quite a bit of Fluoride ions.......as compared to that given off by the salt. The second question you posed is different.....having ZERO to do with a common ion being added.... ....All they want is the small fluoride ion concentration needed to form a saturated solution. I promise you this.....Any Gen Chem text can explain this simply if you need more. I am glad that you do see the slight difference in problem approach......you are correct.

Keep hammering away.

Ok, so I was reviewing my notes and I think I got it. For molar solubilities in something like .1 M NaF, the X's are important because the solid is actively dissolving into the ions. So we do .1 + x. In the precipitation type questions (like the second one), we want to know how much (concentration) it takes to make a solution saturated. A second type of precipitation type question is if the given cxn of two ions would make a precipitate given a Ksp value. These don't require x's (and subsequently an ICE chart), as they aren't really dealing with molar solubility. So a basic Ksp equation like Ksp=[Ba2+][F-]^2 will suffice. Hopefully this helps someone (provided I'm correct).

 

[BYU4you]

Thanks orgoman! By the way are you Nancy or Dr. Romano? I feel weird calling you orgoman all the time

Both, everywhere and always present at the same time.

Generally Dr. Romano for chemistry and Nancy for biology, a rule of thumb but not end all answer haha

 

[pulltheleverkronk]

Both, everywhere and always present at the same time.

Generally Dr. Romano for chemistry and Nancy for biology, a rule of thumb but not end all answer haha

Dr. Romano and Nancy are essentially resonance structures.