EK Chemistry L4 Q91

[fizzgig]

solution to problem states IF the Ksp for BaSulfate and Ag2Sulfate were the same, Ag2Sulfate would be MORE soluble than BaSulfate.

Ksp Ag2Sulf = [Ag]^2[Sulf]
Ksp BaSulf = [Ba][Sulf]

so set Ksp's equal, to whatever val.

val = (2x)^2 (x)=4x^3
val = 👍^2

say 4x10^-9 = val
x=0.001 = BaSulfate Solubility
y=0.000063 = Ag2Sulfate Solubility

so given what breaks into 3ions (2same, 1 different) vs what breaks into 2 ions, if Ksp was the same, the saturation concentration of 3ionthing should be lower, making it LESS soluble than 2ionthing. that intuitively makes sense to me but directly contradicts the solution's text.

sanity checks please.


the next line then says (this is a solubility comparison of which of these would precipitate first given that i'm adding sulfate to the soln) "however, the Ksp for BaSulfate is much lower, so we know for sure that it is less soluble".

the 'however' in there made me wonder. did they mean to say 'if the Ksps were the same, Ag2Sulf would be LESS soluble than BaSulfate, but since Ksp for BaSulfate is so much lower, you know it's going to precipitate first without needing to worry about 2 vs 3 ions'??

thanks people. maybe it's just late...

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[loveoforganic]

You calcualted those molar solubilities backward

 

[UCB05]

solution to problem states IF the Ksp for BaSulfate and Ag2Sulfate were the same, Ag2Sulfate would be MORE soluble than BaSulfate.

Ksp Ag2Sulf = [Ag]^2[Sulf]
Ksp BaSulf = [Ba][Sulf]

so set Ksp's equal, to whatever val.

val = (2x)^2 (x)=4x^3
val = 👍^2

say 4x10^-9 = val
x=0.001 = BaSulfate Solubility
y=0.000063 = Ag2Su Solubility

you have your solubilities reversed in doing the calculations using your val.

x refers to [Ag2Su]
y refers to [BaSu]

x=0.001 = Ag2Su
y=0.000063 = BaSulfate Solubility
Therefore Ag2Su is more soluble at saturation point at the same Ksp

I'm not sure what you're trying to get at with the second part of the post, but maybe this will let it make more sense.

 

[Phantastic]

The answer is saying that if the Ksp's were the same, AgSO4 would be more soluble (this is purely due to the 3 ions); however, BaSO4 has a lower Ksp, so we already know it's less soluble.

I'm assuming you were given Ksp's as well somewhere? I think you understand the problem once you get your variables matched to the correct reaction 🙂

 

[fizzgig]

WHAT. THE. HELL. there aren't caps big enough. sons of beetches. i know it was late and i know i didn't get loads of sleep but i really looked at this for a long time before i posted (maybe it just felt like it). maybe i am too messy and haphazard with my scratch paper. this is so depressing at times.

ok well when i don't label things backwards (over and over) i'm on board.

for what it's worth, can someone help me intuitively with why a 3ion compound has a higher solubility (for a given Ksp) than a 2ion compound? (like a more intuitive explanation than 'look at the Ksp equation'?)

thanks people.

 

[fizzgig]

bumpy? can someone help me intuitively with why a 3ion compound has a higher solubility (for a given Ksp) than a 2ion compound?

wouldn't something with more ions be harder to dissolve/reach saturation faster for a given solubility constant?

 

[Phantastic]

Sorry, I re-read your initial post better. I think you were correct originally, because that "however" would make more sense in your context. (Ag2SO4 was less soluble, however...)

I was too focused on the calculations, but yea, the overall concept is what's important...

 

[fizzgig]

hey Phantastic, so with my calcs redone am on board with yall for the numerical conclusion, but you say "if the Ksp's were the same, AgSO4 would be more soluble (this is purely due to the 3 ions)" ..

1)why does the 3ions intuitively make something MORE soluble than something with 2ions (assuming same Ksp)??

and then (same situation, same Ksp, 3ion dissolved more, both at saturation point: if you added the common ion, i am gathering the 3ion would be more strongly affected (more precipitate than the 2ion compound)??
(from another thread i looked at... don't have link..).
2)why is that? 1sulfate can precip a whole molecule of basulfate, but 2 are required to precipitate Agsulfate, so you'd think it'd have a bigger effect on the Basulfate??


the only way i can make these make sense is
1) given same Ksp, dissolving 3ion vs 2ion is better in terms of entropy. if 2 compounds 'like' to dissolve just as much and 1 increases entropy more, it'll be more favorable.
2)more of the 3ion is in solution so it's more affected.

 

[AliDeLaLouisa]

Looking at this question you just base the answer on the fact of the lowest ksp will precipitate first which is BaSo4 right? The question doesnt ask for calculations just theory? right?

 

[fizzgig]

as i recall, yes Ali - no worries. since the Ksps WERE so far apart, you could just go by that value.

i didn't understand the text of the written solution so i worked out the math (incorrectly at first :/), and now i'm getting a little off topic trying to make sure i understand the concepts behind this stuff. obviously missed this question haha...