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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: AllHallows20 on February 20, 2012, 03:52:54 PM

Title: Which metal will precipitate first given molarities and KSP
Post by: AllHallows20 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

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: Borek on February 20, 2012, 04:14:12 PM

Calculate - for each metal separately - concentration of OH^- needed for precipitation.

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: AllHallows20 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

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: Borek on February 20, 2012, 04:28:45 PM

You are doing strange things.

Write Ksp formula for Ni(OH)₂.

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: AllHallows20 on February 20, 2012, 04:42:25 PM

6.0X10^-16=[Ni][OH]^2

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: Borek on February 20, 2012, 04:47:27 PM

OK, what is Ni²⁺ concentration?

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: AllHallows20 on February 20, 2012, 04:52:39 PM

.200 M

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: Borek on February 20, 2012, 04:58:16 PM

So, what is OH^- concentration needed for precipitation?

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: AllHallows20 on February 20, 2012, 05:02:24 PM

.200 M?

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: AllHallows20 on February 20, 2012, 05:04:13 PM

no wait would it be .4M

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: Borek 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.

Title: Re: Which metal will precipitate first given molarities and KSP
Post by: AllHallows20 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.