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Topic: Which metal will precipitate first given molarities and KSP  (Read 9236 times)

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Offline AllHallows20

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Which metal will precipitate first given molarities and KSP
« on: February 20, 2012, 03:52:54 PM »
This is the question I'm trying to answer:
If a 0.100 M solution of NaOH is added to a solution containing 0.200 M Ni2+, 0.200 M Ce3+, and 0.200 M Cu2+, which metal hydroxide will precipitate first? Ksp for Ni(OH)2 = 6.0 X 10-16, Ksp for Ce(OH)3 = 6.0 X 10 -22, and Ksp for Cu(OH)2 = 4.8 x 10-20.

I know to first calculate when each of the metals will precipitate but I can't seem to get that to work out correctly

Offline Borek

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Re: Which metal will precipitate first given molarities and KSP
« Reply #1 on: February 20, 2012, 04:14:12 PM »
Calculate - for each metal separately - concentration of OH- needed for precipitation.
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Offline AllHallows20

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Re: Which metal will precipitate first given molarities and KSP
« Reply #2 on: February 20, 2012, 04:25:19 PM »
I'm attempting that but I'm pretty sure I'm doing it wrong, I used the formula  ksp=[2.00M][1.00x]^2 for Ni and cu and to the 3rd power for ce

Offline Borek

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Re: Which metal will precipitate first given molarities and KSP
« Reply #3 on: February 20, 2012, 04:28:45 PM »
You are doing strange things.

Write Ksp formula for Ni(OH)2.
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Offline AllHallows20

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Re: Which metal will precipitate first given molarities and KSP
« Reply #4 on: February 20, 2012, 04:42:25 PM »
6.0X10^-16=[Ni][OH]^2

Offline Borek

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Re: Which metal will precipitate first given molarities and KSP
« Reply #5 on: February 20, 2012, 04:47:27 PM »
OK, what is Ni2+ concentration?
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Offline AllHallows20

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Re: Which metal will precipitate first given molarities and KSP
« Reply #6 on: February 20, 2012, 04:52:39 PM »
.200 M

Offline Borek

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Re: Which metal will precipitate first given molarities and KSP
« Reply #7 on: February 20, 2012, 04:58:16 PM »
So, what is OH- concentration needed for precipitation?

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Offline AllHallows20

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Re: Which metal will precipitate first given molarities and KSP
« Reply #8 on: February 20, 2012, 05:02:24 PM »
.200 M?

Offline AllHallows20

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Re: Which metal will precipitate first given molarities and KSP
« Reply #9 on: February 20, 2012, 05:04:13 PM »
no wait would it be .4M

Offline Borek

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Re: Which metal will precipitate first given molarities and KSP
« Reply #10 on: February 20, 2012, 05:13:26 PM »
Why do you guess instead of calculating? Plug Ni2+ concentration into the formula and solve for the only unknown - [OH-]. Basic algebra.
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Offline AllHallows20

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Re: Which metal will precipitate first given molarities and KSP
« Reply #11 on: February 20, 2012, 05:17:05 PM »
oh jeeze, thanks I have no idea how I made things so much more complicated than they had to be.

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