[dagr8est]

Calculate the Br^- concentration required to begin precipitation of PbBr₂ from a 0.90M solution of Pb(NO₃)₂.

***

Ksp of PbBr₂ = 6.6(10^-6)

6.6(10^-6) = 0.90M [Br^-]²
2.7(10^-3)M = [Br^-]

The answer key says it's 7.3(10^-3)M.  I have no idea what I'm doing wrong.  Any help would be appreciated.

[lrarava]

(I don't know how to make those symble smaller on the form. Sorry for the difficulty to read.)
2Br- + Pb2+  <--> PbBr2
c(Pb2)=0.90M
Ksp (PbBr2)=6.6*10-6

[Br-]*[Pb2]= Ksp (PbBr2)
[Br-]*0.90M= 6.6*10-6
c(Br)= 6.6*10-6/0.90M
        =7.3*10-6M
um...the answer key is 7.3*10-3...*1000 my solution...
I also stuck...Explain me what Ksp is please...appreciate it.

[dagr8est]

You have to square the concentration of Br^- because the equilibrium expression is PbBr₂ <--> Pb⁺²_(aq)+2Br^-_(aq).  So if you do that, you would get what I am getting.  I can't figure out what I'm doing wrong and I have a test on this on Monday...

By the way, see the buttons that say sup and sub on the top panel?  That's how you do superscript and subscript.

[lrarava]

here is a link to a similar problem
http://web.umr.edu/~gbert/Aj2.HTML?JAVA/Aksp2.HTM
i still don't understand how to solve this question though
when you get it, please explain it to me
thank you

...just find out it show different question every time...have to paste it...is it fine??
The solubility product constant of barium fluoride in water at 25o C is
Ksp = 1.3 x 10-6 (1.3 E -6).
Calculate the solubility of this compound in water at 25oC.
BaF2(s) = Ba2+(aq) + 2 F-(aq)

solution:
BaF2(s) = Ba2+(aq) + 2 F-(aq)
Ksp = [Ba2+][F-]2 = 1.3 x 10-6
The solubility of BaF2 will be calculated as S moles/liter.
each molecule that dissolves will release 1 molecule of Ba2+ and 2 molecules of F-.
Therefore, in the saturated solution:
[Ba2+] = 1 S
[F-] = 2 S
Ksp = (1S )(2S)2 = (1122)S3 = 1.3 x 10-6
4 S3 = 1.3 x 10-6
S3 = (1.3/4) x 10-6
S3 = 3.3 x 10-7
S = (3.3 x 10-7)1/3
S = 6.9 x 10-3
refer University of Missouri-Rolla webpage
hope it may help you
good luck on your test!

[Borek]

2.7*10^-3 seems to be the right answer.