Solubility question - EK Mini MCAT 10

[pm1]

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It has been a while that I've reviewed solubility and I can't quite grasp the concept behind this.

433. A sample of solid lead bromide is placed in a 0.2-molar NaBr solution and allowed to reach equilibrium. What is the concentration of Pb2+ ions at equilibrium?
A. 1x10^-7M
B. 2x10^-6M
C. 5x10^-4M
D. 1x10^-3M

Answer: D


I remember that one of the ways to approach this problem is through the ICE table. Which would give me Ksp = x (0.2)^2
However, I don't understand how come the solubility of Br in equilibrium remains as what it was already present in the solution from NaBr? Shouldn't the solubility of Br be a little bigger? 😕

 

[chiddler]

hmm don't we need the ksp value to solve this?

 

[pm1]

hmm don't we need the ksp value to solve this?

oh yes, sorry. they give us the ksp value. I'm at work, don't have it with me now. But can you still explain the conceptual of this? Otherwise I will have to wait until I get home 🙂

 

[pfaction]

Then it's just a matter of seeing if the Pb is over the max allowed concentration of PbBr.

 

[chiddler]

oh yes, sorry. they give us the ksp value. I'm at work, don't have it with me now. But can you still explain the conceptual of this? Otherwise I will have to wait until I get home 🙂

NaBr at 0.2 M.

(x)(.2+x)^2 = 6e-6; x = 1.5*10^-4 M

😱

i'd also like help with this.

to answer your original question, mr OP, the reason is that the concentration added by PbBr2 is so small compared to 0.2 that it becomes insignificant. it does indeed change, but not that much.

edit: My question is how is the answer D when I get this number with my calculations?

 

[pm1]

NaBr at 0.2 M.

(x)(.2+x)^2 = 6e-6; x = 1.5*10^-4 M

😱

i'd also like help with this.

to answer your original question, mr OP, the reason is that the concentration added by PbBr2 is so small compared to 0.2 that it becomes insignificant. it does indeed change, but not that much.

edit: My question is how is the answer D when I get this number with my calculations?

okay, that makes sense. I think we are supposed to disregard the +x. Calculate 0.2x =Ksp.
Sorry, I need to get back to you to actually give you the Ksp number. Sorry for bringing it up and not having all the info in hand 🙁

 

[MrNeuro]

don't solubility rules apply here

Pb is insoluble in halides i think

 

[Scalpels]

It has been a while that I've reviewed solubility and I can't quite grasp the concept behind this.

433. A sample of solid lead bromide is placed in a 0.2-molar NaBr solution and allowed to reach equilibrium. What is the concentration of Pb2+ ions at equilibrium?
A. 1x10^-7M
B. 2x10^-6M
C. 5x10^-4M
D. 1x10^-3M

Answer: D


I remember that one of the ways to approach this problem is through the ICE table. Which would give me Ksp = x (0.2)^2
However, I don't understand how come the solubility of Br in equilibrium remains as what it was already present in the solution from NaBr? Shouldn't the solubility of Br be a little bigger? 😕

Howdy, Pm1, I'll see if I can't help ya. Did you ever post that Ksp value, by the way?

I wrote down the procedure for how I'd work it, scanned it into a .pdf document, and giving the link. I may be wrong on how to do it, but I thought I'd give it a shot. As you can see, you didn't need to do the ICE Table.

Good luck!

https://docs.google.com/open?id=0B3k9Pju3ypBkOEhLQnFTeUVCX1E

 

[MrNeuro]

Howdy, Pm1, I'll see if I can't help ya. Did you ever post that Ksp value, by the way?

I wrote down the procedure for how I'd work it, scanned it into a .pdf document, and giving the link. I may be wrong on how to do it, but I thought I'd give it a shot. As you can see, you didn't need to do the ICE Table.

Good luck!

https://docs.google.com/open?id=0B3k9Pju3ypBkOEhLQnFTeUVCX1E

googled the question someone asked the question on yahoo answers

the solution is 1 x 10^-2 ksp = 4x 10^-4

 

[pm1]

hey guys sorry for the delay
the ksp given was 4x10^-5

 

[MrNeuro]

.2^2*x= 4*10^5

X=[Pb]

 

[chiddler]

yeah! that means i wasn't entirely wrong!

i'm curious why wikipedia fed me the wrong ksp data though lol

 

[pm1]

yeah! that means i wasn't entirely wrong!

i'm curious why wikipedia fed me the wrong ksp data though lol

hehe sorry for taking forever chiddler.

 

[chiddler]

hehe sorry for taking forever chiddler.

that's ok. i worked this question 10 times to make sure my numbers are correct, so it helped me review.

thank you!

 

[Scalpels]

yeah! that means i wasn't entirely wrong!

i'm curious why wikipedia fed me the wrong ksp data though lol

lol That's just Wikipedia for ya...

Even when they are using correct formulas and data, whoever writes those articles always seems to go about the longest and most convoluted way of solving examples...

 

[DrRichand1]

Ksp=[pb][Br]^2

4x10^-5 = (x)(.2^2)

x= letter D