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Topic: Ksp Problems  (Read 38826 times)

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Gravemind

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Ksp Problems
« on: March 27, 2008, 02:58:03 PM »
I worked most of these out, I think I understand them all, but I would really like someone to verify my answers.  It would make me feel much better about my upcoming test.

1. Solubility of Silver(I) Bromide is 13.5x10^-5 g/L   What is the Ksp?

AgBr   MW=188   
M=(13.5x10^-5)/(188)=7.18x10^-7
Ksp=[Ag]
=x^2=[7.18x10^-7]^2= 5.15x10^-13


2.  Solubility of Silver(I) Carbonate is 3.2x10^-2 g/100mL     What is the Ksp?

Ag2CO3 MW=276
g/L=(3.2x10^-2)(1000mL)/(100mL)=.32
M=.32/276=1.16x10^-3
Ksp=[Ag]^2[CO3]=4x^3=4(1.16x10^-3)^3=6.23x10^-9


3.  The solubility product constant for Strontium Chromate is 3.6x10^-5.  What is the solubility in g/100mL

MW=204
Ksp=[Sr][CrO4]=x^2
x^2=3.6x10^-5
x=6x10^-3=M
g/L=(6x10^-3)(204)=1.224)(.1L)/(100mL)= 1.224x10^-4 g/100mL


4. The Ksp for Lead(II) Bromide is 4.6x10^-6   What is solubility in g/L

MW=367
Ksp=PbBr2=[ x][2x]^2=4x^3   
4x^3=4.6x10^-6
x=.01047
g/L=(.01047)(367)=3.8


5.  If the concentration of chloride ion remaining in solution after silver chloride has been precipitate is .1M, what is the concentration of the silver ion?

1.8x10^-9=(.1)x
x=1.8x10^-8


6.  A solution of salt contains 2x10^-2 moles per liter of sulfate ion, calculate the concentration of the Barium ion that must be exceeded before the barium sulfate precipitates.  Ksp of Barium sulfate = 1.5x10^-9

Ksp=[Ba][SO4]=[2x10^-2][ x]
[2x10^-2][ x]=1.5x10^-9
x>7.5x10^-8


7. A saturated solution of Lead(II) Iodate in pure water has a Lead ion concentration of .00004 moles/L
a. Calculate the Ksp of Pb(IO3)2

Ksp=[Pb][IO3]^2=4x^3=4(.00004)^3=2.56x10^-13


b. Calculate the molar solubility of Pb(IO3)2 in a .1M Pb(NO3)2 solution

2.56x10^-13=[ x][.1]^2
x=2.56x10^-11

c. 667ml of .435M KIO3 is added to 333ml of a .12M Pb(NO3)2 solution.  Calculate the concentations of Pb and IO3 in the solution at equilibrium

Pb(NO3)=.12x.333L=.03396moles
KIO3=.435x.667mL=.29 moles

Pb+2IO3
.04 moles + .3 moles = .22 moles excess IO3

2.56x10^-13=[Pb][IO3]^2
2.56x10^-13=[Pb][.22]^2
x=5.3x10^-12


8.  The Ksp for Fe(OH)3 is 5x10^-38, calculate the pH of a saturated solution of Fe(OH)3

Ksp=[Fe][OH]^3=[ x][3x]^3=27x^4
5x10^-38=27x^4
x=2.0744
-log (3x) = pOH = 9.2
pH = 4.79

EDIT: Corrected #4,8  (divide, then take the nth root)
« Last Edit: March 28, 2008, 07:52:12 AM by Gravemind »

Offline Borek

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Re: Ksp Problems
« Reply #1 on: March 27, 2008, 03:17:12 PM »
I have just skimmed and looks to me like you are wrong - once ;)

Think again about the last question.
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Gravemind

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Re: Ksp Problems
« Reply #2 on: March 27, 2008, 04:55:32 PM »
The only thing that could really be wrong is my math... or they way I approach the problem, so I double checked the math on that last problem and it looks right so that must mean I approached the problem in the wrong way

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Re: Ksp Problems
« Reply #3 on: March 27, 2008, 05:22:18 PM »
You have approached it OK - it is just tricky. Hint: it is not about Ksp.
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Gravemind

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Re: Ksp Problems
« Reply #4 on: March 27, 2008, 05:43:49 PM »
Did I really just mess up the math?

Ksp=[Fe][OH]^3=[ x][3x]^3=27x^4
5x10^-38=27x^4
x=2.07x10^-1
-log (3x) = pOH = 9.2
pH = 4.79

I took the fourth root, then divided, i guess i shouldve done it the other way.  divide, then fourth root

Offline Borek

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Re: Ksp Problems
« Reply #5 on: March 27, 2008, 05:59:18 PM »
Nope. You are dissolving hydroxide - how come your solution is acidic?
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Gravemind

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Re: Ksp Problems
« Reply #6 on: March 27, 2008, 06:52:43 PM »
Oh... that makes sense, but then why would the math end up giving me a acidic pH....

EDIT: It says on the top of the page that some of the numbers won't agree with the textbook.  I have a feeling my teacher just picked some numbers for examples.

That being said, which way is correct.  Taking the fourth root first or dividing by 27 first?

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Re: Ksp Problems
« Reply #7 on: March 27, 2008, 07:03:39 PM »
Oh... that makes sense, but then why would the math end up giving me a acidic pH...

Because you are ignoring something. Something so obvious you will be banging your head against your desk once I will tell you :)

What was the starting pH of the solution? Before Fe(OH)3 was thrown in?

Quote
That being said, which way is correct.  Taking the fourth root first or dividing by 27 first?

9.2 is correct (in terms of math, not in terms of the real pH).
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Gravemind

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Re: Ksp Problems
« Reply #8 on: March 27, 2008, 07:15:55 PM »
I'm already banging my head against my desk

-log (2.07x10^-11) = pH of 10.7
Which would make it basic... which doesn't make sense so I'm wrong again....

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Re: Ksp Problems
« Reply #9 on: March 27, 2008, 07:23:58 PM »
Answer my question from the previous post...
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Gravemind

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Re: Ksp Problems
« Reply #10 on: March 27, 2008, 07:33:14 PM »
What was the starting pH of the solution? Before Fe(OH)3 was thrown in?

It doesn't really say.  All it says is that it is a saturated solution so it should be very basic

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Re: Ksp Problems
« Reply #11 on: March 27, 2008, 07:39:58 PM »
What is pH of pure water?
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Gravemind

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Re: Ksp Problems
« Reply #12 on: March 27, 2008, 07:53:45 PM »
7

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Re: Ksp Problems
« Reply #13 on: March 27, 2008, 08:01:38 PM »
So there was already 10-7M OH-... Dissolving Fe(OH)3 will increase this concentration.

1 am here. Good night.
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Gravemind

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Re: Ksp Problems
« Reply #14 on: March 27, 2008, 08:07:58 PM »
Oooo, I think I get it now.

Good night, and thanks for being so patient with me :)

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